mini mac

You will need to get and use an external monitor (display) with the Mac Mini.

see > Connect a display to Mac mini - Apple Support

Your 2015 iMac does not support being used as a display in Target Display Mode,

plus there are no new Mac's including the Mac Mini can use a qualified iMac as a display.

please carefully read > Use your iMac as a display with target display mode - Apple Support

if it's actually a 21.5 inch monitor, it should work fine. but if you mean you have a 21.5 inch iMac, then, no, that can't be used as a monitor for the Mac mini.