Oh my God, are you effin' serious? There is so much wrong with this statement, I don't now where to begin. The idea that there could be a clown kingpin of crime, or a DA turned insane half-man/half charred corpse, or a deranged psychiatrist who dresses as a scarecrow and drives people nuts with fear gas, or a plant woman who bewitches men with her spores, or ANY of that stuff is pretty far-fetched in a real world sense. You aren't going to be seeing any of this on the nightly news. It is based on comic books. So to talk of the Joker's transformation via a bullet, a betrayal, and some funky green chemicals as if it is too lowbrow for today's audience is absurd. Even when the movie first aired, nobody thought "Oh yeah, that's completely plausible, happens every day." It was a plot device, a convention that audiences accepted to get from "point a" to "point b" in the movie. And the scene with the back alley plastic surgeon and the Joker smashing the mirror is still awesome - whenever I see it, all I can picture are comic book panels with the dialogue: "Hee hee hee hee hee! A-hee hee hee hee hee! Ah ha ha hah hah HA HA HA HAW HAWWW!!!" scrawled in crazy lettering across them. And that's what 89 Batman was all about, being a dark, motion comic book.
Oh, and before you completely poo-poo the mystery green chemicals from 89 Batman, in the 1988 one-shot comic "The Killing Joke", the Joker's back-story involves a chemical-engineer-turned-failed-stand-up-comedian falling into a vat of chemicals during a crime gone wrong and being transformed into the bleached white Joker. Anything sound vaguely familiar? I think you'll find it was this comic that influenced Burton's Joker genesis storyline, much like "Batman: Year One" and "The Long Halloween" have been influences on Nolan's take on Batman. These movies are ultimately a retelling of the comic book tales that are their inspiration - and either your imagination can embrace these ideas or it cannot.