That’s the proprietary app container system pushed by Canonical who maintains Ubuntu. That’s as opposed to something more widely accepted like flatpak. I’m not an expert on everything Canonical has done to piss of the FOSS community, but I think snaps are the biggest one.
And in regular old Linux Mint Cinnamon you don’t have to deal with that, and you can still lean on Ubuntu’s apt repositories.
I’m good with cinnamon myself, but you can totally install KDE on mint instead. It is just Linux after all.
When I do a search for mint kde, this is the first result and has step by step instructions. It looks like it can be as simple as apt install kde-standard or kde-full.