They have a version (LMDE) based directly off Debian. It's slower to get updates (because Debian) but it's pretty much the same apart from that.

I really wouldn't call it a reskin of Ubuntu. It uses the Ubuntu repositories sure, but it also has many of its own packages and there is the LMDE version which bases on Debian instead of Ubuntu (in case they ever want to become independant of ubuntu). Mint has none of the crap Ubuntu comes with and won't inexplicitly install snaps.

Mint develops their own desktop environment cinnamon (based on gnome 2).

If a distro is a reskin one should be able to convert an install of the original distro to the reskinned version. Mint comes with heaps of its own software, for managing software installation, software upgrades, managing kernels, managing release upgrades and of course its own DE.