I like the look of both of Burton's Batmans, but particularly enjoy the darkness of the second one, so no, it is not too dark for me. If he was actually dissed by the studio, then they were mistaken. Burton brought us a comic book Gotham for sure. I would have watched another Burton installment, but as it was, I did not see any subsequent Batman release.
Michelle Pfeiffer was very good, as was Danny DeVito, as was Christopher Walken, as was Michael Keaton. I enjoyed what each of them brought to their respective roles. As much as I liked Jack Nicholson's powerful persona in his portrayal of The Joker in the first film, it was more enjoyable that Batman had several adversaries in the second one.
To answer your question about Pfeiffer: she was excellent at portraying the playful, vengeful, cold, sexy, crazy kitty that she was. I can't really think of another actress of that time that would have done it better.
Too dark? I always thought it was too over-the-top to be anything but very silly. De Vito, Walken and Pfeiffer all ham it up to ten and the whole thing turned into something quite weird. But I never thought of it as dark... that is... until the next films arrived (especially the Clooney one) and they were so kiddie friendly that by comparison, I guess this one is kinda dark.
I remember it just being ridiculous, to be honest. And for Michelle Pfeiffer in that leather suit. Ding dong.
It's certainly not my favorite Batman film. Leagues ahead of Schumacher's films, naturally. It was dark in places, but I also thought it brought a proper comic book Gotham to the screen. The only thing I still hate is DeVito's Penguin. For me, he was the heavy guy in a tuxedo with a penchant for birds. Here, we get a deformed monster out of Burton's masturbatory fantasies. Other than that, I dig the movie.