Eh. Not quite. Yes, the main editions use Ubuntu as a starting point, but they remove a load of Canonical’s cruft, like Snaps. They have their own suite of applications, the XApps, that are forks of other tools, as well as a number of other improvements and changes.

I couldn’t say whether it’s as far from Ubuntu as Ubuntu is from the original Debian, but it’s some distance removed for sure.

And LMDE is based on Debian, skipping Ubuntu entirely.