I really wanted to like this film. I love horror, and I especially love horror satire and parody. And I can usually cut writers of parodies a lot of slack when something isn't outrageously funny. I write parody humor, so I know how hard it can be to hit your mark. But nonetheless, this was pretty bad.
Of course, the urge to compare it to Scary Movie is very hard to resist. It simply boils down to the fact that the Wayans were trying to add some wit to Scary Movie in addition to making fun of horror movies. The makers of Shriek..., on the other hand, thought that simply visually copying a scene from a horror movie is inherently funny on its own, with no need to write actual gags to go along with it.
And of course, all of the overused cliches in parody humor are present...being a lesbian is supposed to be funny, being a virgin is supposed to be funny, drug use is supposed to be funny, being overweight is supposed to be funny, etc. But again, they are simply presented, and not used to incite humor. Asking a Wiccan-esque girl if she was a lesbian *may* have been funny on its own, say, a decade ago when every high school didn't have its own Wiccan chapter, but now it takes a little more insight to make that stereotype funny.
Even the one more-recent cliche, being a big-busted woman actually having implants is supposed to funny, is rolled out almost as an unfunny non-sequitur. The exact same joke is done as an actual punchline in Scary Movie. But as I said, I'm trying to avoid comparison.
You can just tell the writers and director didn't really know how to handle this style of comedy. They obviously are trying to emulate the movies of the Zuckers (you know you're in trouble when a character in a parody mentions a funnier parody), but at the same time they didn't understand what made them hilarious. It's perhaps the cinematic equivalent to listening to a Beatles cover band. You can tell they know the music and have a real love for what they're doing, but there's no magic behind it.
The writers even try to explain what they feel is the parody "formula" by having the lead girl list the "rules" to survive a parody. The rules she gives seem more fitting for a Problem Child sequel or a fourth-season episode of Ren and Stimpy. As far as I know, Airplane! didn't have Hanna-Barbera sound effects or misleading sex shots.
The movie seems to have overlooked the only real rule to survive a parody: Make it Funny!