Of the "Film Crew" movies, I first saw Wild Women of Wongo, probably two months ago now. It's one of those very vaguely exploitative movies from the '50s, which probably seemed terribly racy to some people at the time, but which are ridiculously tame now. In an unspecified prehistoric land, there's a village with gorgeous women and homely men, and another which is vice-versa. As you would expect, they meet, and predictable problems ensue. (There's also a priestess in that odd Florida tourist-trap made of huge hunks of carved coral, and a mangy alligator, but that pretty much sums up the characters and plot.) The commentary gets good in spots, but the movie is so dull in its outdated prurience that it rarely gets up much speed. The whole package is OK, but a bit disappointing.
Killers From Space should be even more tedious, since it has, if anything, less plot. (Test pilot Peter Graves is thought dead in a plane crash, reappears with amnesia at the edge of town, and then weird aliens appear.) But these guys have an odd Peter Graves comedy fetish, and his appearance always ups their game -- though they do rely on a lot of Biography jokes during this one. So the movie itself is deeply tedious, with long scenes of Graves running through the aliens' underground base, but the commentary is excellent during those scenes, which makes the whole package very enjoyable. (The film also jumps to random close-ups for no good reason, which the commentators get some mileage out of.)
And the "Film Crew" movie I liked best was Hollywood After Dark, a dull "this town will eat your soul" movie featuring a somewhat younger Rue McClanahan as the ingenue (and as a stripper, I warn you now). Again, the movie itself is absolutely tedious and dull, in often inane ways -- there's a fifteen-minute caper sequence where the soundtrack goes completely silent, for one thing, plus the usual issue of not having enough plot for the length of film -- but that leads to very funny commentary.
There could be a general rule of riffing there: the more inept a movie is -- badly done on the fundamental levels of lighting, framing, sound synchronization, and just plain aiming the camera at the right thing -- the better fodder it is for making fun of. There are plenty of movies with plot problems, so jokes about logical holes can only go so far. But if the things you see on screen just don't work, that's rarer, and thus more funny. (Call it the "Torgo" theory of bad movies.) Unfortunately, that theory does not explain why Mitchell is so damn funny, since it's very technically professional. Perhaps there's an additional Joe Don Baker effect?