Photos taken on an iPhone are good, unless you:
* zoom in
* print them
* watch them on a bigger screen
Sometimes I compare photos I've taken over 10 years ago with Sony NEX-5 with photos I take today with an iPhone. There's no competition, APS-C from 15 years ago is still solid.
Anyway, the best camera is the one you have with you, so in that sense iPhone is great.