screen for mac mini

That will depend on what iMac and what Mac mini models you have.

If you have a iMac from 2009 to 2014 and a Mac Mini released on or before 2019, then yes.

click here ➜ Use your iMac as a display with target display mode - Apple Support

If you have any other combination of iMac and Mac Mini, probably not.

Not in any way that performance would be acceptable.

And the Mac mini will require a regular monitor in order to set it up.

You'd be wise to budget for a monitor to go with the mini.