I would like to chime in as the Mumblecore Defender because yes, any description of the fims sounds really miserable and a lot of them are! However, a lot of them I think are really worth watching.
Most important thing about them is they exist in a really interesting spot between 90s-00s independent film and the rise of easy to use video production technology. So you get extreme democratization of movie making (anyone can do it!) and really interesting experiments with improvisation (a lot cheaper to keep rolling when you’re not paying for film by the foot).
The obvious outcome is that scenes are meant to be more naturalistic than had been seen before. Interactions are low key, rambling. Plot points are slipped between a lot more everyday interactions. The other major outcome is actors are very involved with characterization and choices for their roles. My wife likes a lot of roles for women in these movies compared to other comedies created by men because the actresses clearly have a lot more input into how these characters exist. There also arguably was more room for women in non-acting roles as well, though it’s still dominated by white men.
Also, they’re funny, to me. They have good jokes in them and believable, funny interactions. They’re not just navel gazing.
Fave mumblecore director is Andrew Bujalski, who is weirdly now directing the live action Lady and the Tramp remake. He’s the most plainly talented of the big names though the Duplass brothers are much more famous at this point. My favorite is Computer Chess, some may not call this mumblecore but that sort of thing is expected with any sub genre I think. The IC community may be particularly interested in the fact it was entirely shot on Sony AVC 3260.
I had not heard the term Mumblegore until today but I can say the Creep and Creep 2 own and House of the Devil is also good.