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.