What kind of app are you talking about?
I don’t see much of a point in using UE4 for that, unless you need to render something in 3d.
Also it depends from app to app, if you need to make something like duolingo then yes, because duolingo is basically a game.
If you need to access some hardware, like the camera, or turn on the flashlight, then it will be impossible without modifying the source code.
Like you’re saying you want to show menus and images, if the images are in the app and not in a separate folder of the phone that it’s doable of course, it would be the same as displaying UI in a game, just without the game in the background, mostly with the engine you’ll be limited to things you have in the app when you compile. If you need to load, save or access things that are not compiled in the game then it’s gonna be a pain, with any game engine.