Installing a second DE (KDE) on top of Mint (XFCE) is technically fine, but often messy. You end up with duplicate apps (two terminals, two file managers, etc.) and potential config conflicts.
If you want stability + a clean experience, I'd recommend backing up /home and doing a fresh install of Kubuntu or Fedora KDE.
But if you just want to try it: 'sudo apt install kubuntu-desktop' works. Just expect some 'bloat'.