I don't mind the bad movies if they're also cheesy, plus the old ones are so innocent. I watch MST3K late at night, so if I watch Monster a Go-Go and chuckle a few times and then fall asleep that's fine. What I don't like is the sleazy bad movies like Manos (very likely my least favorite MST3K), TISCWSLABMUZ, Hobgoblins. Blech.
Usually though, the thing that will put me off an episode is the riffing. I don't mind sparse riffing at all, I have no problems with Season 1 and I like KTMA a lot for its movie choices and fun improv feel to the light riffing. But, when the riffing is lazy or bad, I might not come back. The Sci Fi seasons are much worse at this. There are episodes where Mike and the gang just come off as lazy and sometimes mean spirited. I can't stand the Final Justice ep because roughly half of all jokes are making fun of JDB, calling him fat and talking about eating. And, not the character either, but JDB. Lazy AND mean spirited. Blech.
They also go overboard on single topics often in the Sci Fi years. I used to think Prince of Space was a funny ep, but on repeat viewings it really started to annoy me that half of all jokes are just Japanese culture jokes, many not very good or clever, just shouting out Japanese culture in a negative light, like bringing up tentacle porn, more than once. Blech.