We watch a ton of B movies and i saw someone make this graph. I dont remember where, so i made my own. But a good b movie is one thats so good its bad. Most B movies are just boring. The redlettermedia guys describe it perfectly. The producers and actors play it straight and try hard to make a good movie. And they fail completely.
Edit: that quality axis should go the other way. Good quality is on the left not the right.