Get gross, not gory.
Lesson learned from: Re-Animator (1985), Dead Alive (1992)
OK, so the jokes are legitimately funny and the horror stuff is authentically frightening. Got it. But how frightening do you get, and by what means?
If you're ever trying to traumatize your friend, go rent the 2007 French knockout Inside, which has some incredibly tough-to-watch imagery of the dismemberment and unsanitary-C-section variety. You'd have to be the real-life version of Henry: Portrait of a Serial Killer's main character to find anything in Insidethe slightest humorous. Carving a living woman's womb open is about as funny as a Whitney Cummings joke.
Putting carnage of that intensity level into a horror-comedy isn't gonna work. Keep in mind, the point is to allow viewers to enjoy themselves and chuckle as much as they shiver.
The solution: Make the gore cartoonish—there's no such thing as "over-the-top" in a horror-comedy. The more outlandish the visuals are, the easier it is for audiences to accept the ridiculousness and laugh it up. Case in point: Peter Jackson's magnificently gross Dead Alive, a zombie flick where an old woman eats soup with the key ingredient of "decrepit zombie ear," the hero goes chainsaw-happy in one of the goriest-yet-silliest sequences ever put on film, and large monsters pop out of small human orifices.
You can't help but laugh at something that absurd.