So-bad-it's-good film isn't the worst film in every dimension—often it's competently- or even well-made in at least some ways. Films that are simply all-around bad, made with no amount of skill at the craft and insufficient effort, usually aren't entertaining and aren't the kind of thing anybody wants to watch. So-bad-it's-good is defined by being a kind of bad that one can still appreciate, even if part of the appreciation is of the ways in which it is bad.

There was a thread on here about bad songs the other day, and the kind of bad people meant wasn't, like, an untalented and under-practiced 9-year-old screeching out their original composition on a violin. Obviously that's worse than nearly anything, but nobody means that when they talk about something like "what are the worst songs?" A credible effort has to be put in for anyone to even care to think about it to shit on it.

I think it's still useful to call those categories "bad", even if they're not the most bad. Often the badness is what distinguishes them from the merely forgettable.