Scary Movie 5 is not a spoof. It’s not even a comedy. It’s a mess of very vague references to mostly outdated cultural phenomena, sprinkled in between half-hearted and half-baked “gags.”
Genre spoof is supposed to expose the conventions of that genre in a clever way.
The original Scary Movie was successful because it came on the heels of a horror film Renaissance that began with the movie Scream (which, in itself was somewhat of a spoof, but, that’s okay).
Scary Movie had a reason to exist. It had something to say.
Mainly, “Boy, there sure are a lot of horror movies copying Scream these days.”
But what is Scary Movie 5 saying? I sat through it, and I couldn’t tell you.