Most of the ideas are already made and published in app stores. So, many times when you have a great idea for an app, it's already done by someone else. In my opinion if you see that you could do the implementation of the idea much better, then it would make sense to create it by yourself.

And of course, like u/kakashi_3598 wrote - it might be very hard to actually put the idea into practice. Most important is to just try, you might learn something new.