The Joker isn't meant to be haha funny or whimsical - he's a sociopathic murderer, not a comedian. Frankly it's the most true-to-form portrayal of the Joker to date.
"Dark" Batman isn't any more valid an interpretation than "campy" Batman. You don't have to like both versions (I certainly don't), but they've both got a pretty extensive history. I don't think you can really claim that one version is more "true-to-form" than the other.
Besides, you can be a sociopathic murderer
and a comedian.
On the other hand, though, I can't deny that it
was stupid of me to go to the Dark Knight knowing that I only like campy Batman. I was swayed by everyone I know loving this movie.