I always have a hard time with this question.
Sometimes movies are intentionally going for a campy/fun tone, such as 80s comedy-horror movies like Tremors and 90s action films like Big Trouble in Little China.
Are they really "so bad they are good" when they are not taking themselves too seriously, self-aware of their B-movie status, and just going with it to great effects?
I usually try to confine this to films lacking irony or self-awareness.
It is just hard to know what they were going for sometimes, and sometimes the history of a bad film (e.g., Batman and Robin) is rewritten as, "Oh, we totally meant it to be silly and fun all along, believe us!" when that might not have been the original case at the time.
Was Flash Gordon trying to be the next Star Wars (or rather the original SW) or just having fun?