B-movies are always easy to parody because they're so bad and cheesy to begin with. Therefore, you can go with low-budget "talent" who aren't good actors, cheesy special effects, and a bad script. It's basically shoddy filmmaking, but it's often quite fun. The prime example of a B-movie spoof is Killer Klowns from Outer Space. The title clowns come down from space in a giant circus tent, where the obligatory farmer with his dog goes out to investigate it. The teenager, with his girlfriend, find out the truth (that killer clowns from outer space are transforming humans into cotton candy cocoons so they can drink their blood with a curly straw) and go to the police, where the curmudgenous cop (John Vernon) doesn't believe them but the younger cop does, and they go out to stop their small town from being terrorized.
Killer Klowns from Outer Space is a silly movie, to sum it up. The clowns shoot popcorn at the townspeople and delight them before devouring them or shooting them into cocoons. Obviously, this is just supposed to be plain fun, and it is quite entertaining. I'm glad to see no one takes it seriously. If anyone did, I'm sure they hate the B-movies of the 50s, also. The one difference, though, is that those 50s movies weren't trying to be funny, while Killer Klowns was. It tried to copy the dialogue that made the 50s movies work, and occasionally succeeded. But the only time I remember laughing was at the hand shadows. The clowns, too, didn't look like the way they would look. These aren't the images people get when they think of clowns.
Vernon saves the movie with his deadpan role with which he's having fun. The rest of the performances are what you'd expect-they're like the B-movie actors. Killer Klowns from Outer Space isn't an Airplane!-sized spoof, but it's cheesy and, for the most part, entertaining for what it is. It's impossible for anyone to think more of it. It does have a great tagline, though.