Can a 2015 iMac be used as the main display for a Mac mini?

Target Display Mode went away when the first 27" 5K Retina iMac came out in Late 2014. Your 2015 iMac doesn't support it. Apple later added restrictions such that current and recent Macs do not support using any iMacs (even very old ones) as Target Displays.

No! There are numerous threads in this forum that explain why so I won't get into that. Bottom line is if you want to use a Mac mini then you need to purchase an external display. They start under $100 so cost should not be an issue.

+1 👍🏽