It can be considered slightly inferior depending on your viewpoint. Debian tends to have older versions of software components than Ubuntu, and there’s some Ubuntu tools that are quite handy (the graphics driver management tool is one I believe).

So, in my understanding there are good reasons they don’t just pivot to Debian. I am not on the Mint team so you’d have to talk to them, but my recollection of a press realease or interview from a few years ago was that bringing all the ubuntu stuff into LMDE and maintaining it would be a massive effort