Can I use a IMAC 2012 as a monitor for a Mac Mini?

Depends on what model of Mac Mini you have and what version of Mac OS is on it

The mac mini needs to be from 2019 or earlier and have mac OS catalina or earlier

See:

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

iMac models introduced in 2011, 2012, 2013, and mid 2014

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Thunderbolt  or Thunderbolt 2  cable.

While the 2012 iMac will work as a display in Target Display Mode for some older Mac’s, it will not work for any new Mac or Mac Mini.

For the full Target Display Mode spec’s and requirements, carefully read > Use your iMac as a display with target display mode - Apple Support

Most likely not, good news is a normal monitor use vastly less power than an iMac in target display mode

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