Gotham City itself was surreal looking. There was no real city that looked like it. Once that film started you knew you were in for a gothic fantasy. It was the 80's man the decade of style over substance the whole purpose of the film and Burton's M.O. at the time was surreal visualism. Now anybody who cared enough about Batman outside the movie would make it their business to know what the deal is anyway.
Chances are anybody who goes off what the '89 movie stated doesn't care enough about the mythos in the first place to do their research so really who gives a damn what they think as long as you as a fan know the truth.
The only reason the chemical bath is highly regarded as canon by a lot of fans including Ed Brubaker who wrote it into his Man Who Laughs one shot is not cause of Burton's movies, it's not even cause of The Killing Joke. It's because of this comic right here
Had that issue never been published people wouldn't really be running with the idea for many decades after it's release. Mind you I don't care for a Joker origin myself I prefer an ambigious background I don't like when too much is spelled out for the audience. However to think that people just like the chem bath idea cause of Burton's film is a little silly. People just liked that idea because it's been the most consistently played with out of all the possible origins or perhaps because they want to come across as purists but not because they can't get over that movie from 18 years back.