The sudden change in direction really ruins the movie for everyone. The character Marla literally does not have a single redeeming characteristic and is nobody even the SJW crowd would want.
Worse, all the wildly improbable plot elements which should have been funny suddenly have to be taken seriously.
There's a scene for example where Marla is chloroformed, has half a bottle of Vodka pumped into her stomach via nasogastric tube, is placed in her car with the throttle jammed wide open and launched down a hill towards a river.
Not only does Marla magically wake up with all her faculties intact, she suddenly has the superhuman strength to knock the back window out of the car and escape without so much as a hangover.
Villains who refuse to die are not uncommon in comedy, (Jaws from James Bond) the supernatural, (Samara & Chucky) or science fiction (The Terminator) but it just doesn't work here.
The audience is left floundering for a plausible explanation, with some even speculating that Marla really didn't survive and everything afterwards is hallucination as she drowns.