Success is entirely subjective and dependent on too specifics unique to your situation. For example, are you currently pursuing your app as a hobbyist, hopeful it will augment your spending money but not expecting it to replace your annual salary? Or are you expecting to raise outside investor money based on this definition of success? Even then, the metrics for an enterprise app versus a game, versus a utility are entirely different, regardless of what your financial expectations are.

So given that none of us who can answer this question for you know enough about what you're doing, I can give you the most helpful generalized advice: Outstanding success of any mobile app, should be defined by user love. Of course, love can take many forms from addictive ("I can't quit you") to bliss ("everytime I use this, it just makes me feel better") but the sign of outstanding success of a mobile app, is that it is widely loved by the people that use it. They could use it once a day, a week, a month or whatever (again context matters), but that every time they use it, they love it. When users love something, they tell the creator, they talk about it publicly and on social media, and it's generally well reviewed (there can be exceptions).

So you could define "moderate success" as being liked, but I really don't know why anyone would want to "settle" for moderate success. The experience of being widely liked is probably best experienced by a group of users going "man, I like what you're doing, but when are you going to do this, or I'd like it even more if it did this" If you're getting that type of feedback from a good portion of users, then you know you've still got a chance to get to being loved.

Define success of your app by user love. Not even reviews (though they can sometimes be a barometer for love), pure love.

Happy to talk to you in a quick call. I'm sure that with the missing pieces of information from you, I could quickly answer the question you're asking with the specifics you seek.

Success should be defined relative to your category. If you're marketing a game your criteria will be different than if you're launching a 99-cent weather app. For example, take a look at download numbers required to enter top 25 in a few different categories:
http://thechocolatelabapps.com/downloads-app-store/

Your strategy to get there will have to also depend on your vertical, but generally it involves a mix of paid user acquisition (via mobile ad networks), organic acquisition through social, viral and web, and finally look at potential for dominating App Store search (ASO).

It is easy to gauge your chances - ask your target customer. The process is simple: Define who would use your app and why. The "why" is the hard part. If you can build something of value to a group of users, success happens. If you find something that is causing them pain and build an app that alleviates that pain, success happens. If you start with the "why" the rest flows. If you start with the "app" and your isolated thoughts about why YOU think it will be successful, it usually doesn't end that way. Start with the "why" then answer the "how".

To access chances of successful mobile app yet to be launch, keep the following steps in your mind:
Step One: Know Your Audience.
A successful app launch strategy is firmly rooted in user research. There can be multiple groups of users for an app, so during the early planning and discovery stage of a project, you must identify the product’s central demographics and create user personas. A user persona is a semi-fictional representation of the product’s ideal user. The more detailed user personas are, the better. Demographics, behaviour patterns, motivations, and goals are just a few aspects to include in a user persona. A user persona is one of the most valuable tools in the user experience design field. A central benefit to user personas is each personal narrative keeps the team focused on a validated product outcome and specifically prioritizes how the user will perceive that outcome. User personas also combat scope creep and reduce self-referential design decisions. Detailed user personas will supply answers to several key questions to guide your launch efforts and set the foundation for your entire app launch strategy. Put simply, if you do not have a solid understanding of who your audience is, you can’t target users properly with your launch strategy.
Step Two: Define Your Positioning Statement
This is called a Unique Value Proposition which describes why your product is valuable to your customer and why they should buy-in. Your message will be drowned out if you simply describe what your app does. Instead, focus on why it is different than any other product on the market. Additionally, it is imperative you ensure that your internal team is on the same page with your messaging understanding the intent behind your product. Strong internal communications is an essential part of a mobile launch because if you aren’t able to communicate effectively to your internal team, you won’t be able to properly communicate your message to your external stakeholders and in turn, won’t be able to achieve the anticipated buy-ins.
Step Three: Define Your Success Criteria
Success criteria outlines the benchmarks that your product will be compared against to judge if your app launch has been successful. To show stakeholders key information on your success or failure you need to first set some goals and define how they will be measured. Set your goals around active install rates, number of reviews, and average rating score, for example.
Step Four: Prepare Launch Activities.
It is essential to identify industry influencers who are actively engaging in conversations related to your industry or are talking about other apps related to your industry and target users. Network and connect with these individuals, either through networking events or by connecting with them via social networks. Craft a persuasive pitch that will convince influencers to promote your app. To maximize your odds of getting press coverage, make it as easy as possible for bloggers and influencers to feature your app. Compile all your assets in an online directory so they have easy access to the information needed to promote your app. Put careful consideration into how you will craft your product description to convince potential users to download your app. This means you want to pique interest in the first few lines of the description. Get your users excited about downloading and using the app. The higher your app ranks, the more visible it is to potential users. This is a great way to organically boost your app marketing and promotion efforts. Strategically place keywords throughout your listing and description so you show up when users search for those words. Make sure this is clear in the description of your product. Mention a few key features in the description as well, and let users discover the rest of the app after they have downloaded it. After you have planned all your launch campaigns, assess what resources you will need such as demo decks, product screenshots, design materials, and other design resources.
Step Five: Launch Your Mobile App.
At this stage, you should be aware of the submission guidelines your app needs to meet before submitting to either app store. Apple’s guidelines can be found here: https://developer.apple.com/app-store/review/guidelines/
and Android’s guidelines can be found here: https://developer.android.com/docs/quality-guidelines

Step Six: Post Launch Follow-Up
If you do not impress your new users quickly, you will likely lose their interest altogether. Some reasons why users abandon an app are the lack of interest, change in user needs, and poor user experience. Ensuring that you update the app experience and add new features and personalized content regularly will help keep users interested and engaged. By using analytics, tracking user behaviour, and listening to user feedback, you can determine what your users want and what features of the app are driving usage, which can be used to inform product road mapping and ensure the updates you make are compelling and valuable to your users. When updating your app based on user feedback you need to consider if this feature meets your business objectives while adding value for your users. One of those methods is to give users an incentive to use your app. All of these will give users a reason to choose your app over another and will help drive conversions and encourage engagement. Of course, the ways you can incentivize users will depend on the nature of the app. For example, apps that use in-app purchasing as a monetization model will benefit from time-sensitive discounts, whereas freemium apps can incentivize users by providing usage-based rewards. Push notifications are an effective way to encourage re-engagement for users who have dropped out of the conversion funnel. However, users do not want to be bothered with irrelevant messaging so be thoughtful and strategic in how you reach out to users. Developers should encourage their users to submit reviews, be diligent in managing and seeking to resolve negative reviews and should keep communication channels open to field user feedback.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath