There is a theory going around on the internet that Joker is not meant to be THE Joker, for the reasons you describe. If that is true, this film is simply about a psychotic loser.
Film-makers seem unable to resist sticking to source material. Sometimes that actually improves upon it while they're tasked bringing printed or graphic/comic characters to three-dimensional reality while suspending disbelief. Sam Raimi had the webbing shoot straight out of Spider-Man's body and it didn't hurt those films at all. Letting other directors handle the third one did.
"The Dark Knight"'s Joker never fell into a vat of skin-bleaching chemicals - just wore make-up over facial scars for intimidating effect. And that worked just as well on Heath Ledger throwing his entire soul into that portrayal. He could have acted that out with nothing on his face and still creep me out. THAT Oscar found it's proper home.