There's nothing wrong with a comic book character acting like a comic book character, just like there's nothing wrong with one trying to be more than that. Even that statement is wrong, because it implies comic books are only for kids and stupid stuff, which is totally wrong. Stories like Watchmen, Sandman, The Dark Knight Returns, Year One, Long Halloween, The Cult, Arkham Asylum, Miracleman, etc, are there. I don't know when was the last time you read a comic, but it has evolved a LOT since. Characters and stories have evolve. Saying they didn't is lying.
These characters are complex and their stories can be explored and diverse. If we refused to evolve characters from what they are, then we will never have something new. That's not how art works. Just look how many rich stories we get when we do that. If Nolan didn't think about that, TDK trilogy wouldn't exist.
And Joker being in kid's cartoons doesn't make him a less serious character. If nothing, in BTAS he's the most sickest and evil character and problaby the best villain in a cartoon show. Also, characters like Deadpool and Punisher appear in cartoons. Does that make them kid's characters?