What version of iOS are you running? My 12 minis were like that for awhile but the last version of iOS, 14.8, seems better. iOS 15 on my 13 Pro has some quirks.My iPhone 12 mini is like that- gets noticeably warm on the back with the most basic use, and on random occasions gets downright hot.
Didn’t start doing that until a couple months ago— I’ve been thinking if the 13 series run cooler, it would be worth an upgrade.