It is possible to market an app in a small niche for a certain group of people and their specific needs or go broad. A micro niche won't have lots of traffic but will have higher conversions. In the second case, an app can provide a larger pool of potential customers and more fierce competition.
In any direction, uncovering people’s pain points ensures real value and can make an app successful. These users are easily attracted and converted into loyal customers.
The key is to be open, listen, and ask people. Study already existing products, and surf online resources and social media for the information. Note that to ensure an application’s success, it is important to define what inspires or irritates your target audience.