To me, a bad bad movie has nothing going for it, like "Robot Monster". Gorilla with diving helmet walks around and does nothing. Family walks around and does nothing. Nothing happens...
A good bad movie has lots going for it, even if it's laughably dumb, like "Plan 9 from Outer Space". The Solaranite scene alone puts it head and shoulders above just about every other bad movie! For all the laughs and fun it provides, I would never put it on a 'worst movies ever made' list like so many do.
Some of these types of movies cross over into just plain good movies. ;) I speak of the original versions, not the MST3K'd ones. Those should be viewed on their own merits, as many think that they can turn a bad bad into a good bad.