Instead of asking "Does it help people understand publicly available health data?", a better question may be "In what validated scenarios does public health data contain the answer to people's questions/problems/needs?"
The target of the app is young adults who are looking to move to a new
area, and may be concerned about info like smoking rates, allergy
alerts, HIV/AIDS rates, etc.
...seems like an assumption that needs to be validated. Are young adults concerned with this data when moving into an area? The success of the app rests on the truth of this assumption.
This is a question properly answered by doing user research before creating an app. Here's a few good thoughts from a recent article on UXMatters: How to Know When Your Product is Going to Fail
A primary purpose of UX research is risk mitigation. When we perform
research we often look for indicators that let us know whether we are
on the right or the wrong track with product development—and, if we’re
on the wrong track, how we can get back on the right track.
If you put a product in front of users and they don’t tell you that
they want it, you might be on the wrong track.
At the current point in your development cycle, it could it be as simple as: "Usefulness is indicated by usage" - analytics will tell you how useful your app is. If people do not use an app or continue to use it (providing it receives enough promotion/publicity) it is an indication of a flawed user need scenario.
The concept of validating assumptions (and user interest) early is a key idea in The Lean Startup. If your assumptions do not validate, it does not necessarily mean that the entire idea is faulty, but that a pivot may be needed to a different approach that better connects the useful data to actual user needs.