Using an iMac to run a Mac Mini

According to your tag line, your iMac is a 21.5" iMac running macOS 15.7 (Sequoia). The only 21.5" iMac that can do that is the 2019 model.

Target Display Mode went away in Late 2014 when the first 27" 5K Retina iMac came out. So your iMac is far too recent to support it. Another issue: Current Macs do not support using any iMac as a Target Display.

no, at least not as it's only screen

Use iMac as monitor: 5 Solutions - 9to5Mac

and performance would not be like a real monitor and

it's power consumption would be much much higher than a real monitor.

Nope' plan on using an after-market external display with the Mac mini.

Connect a display to Mac mini - Apple Support