I agree that the direction of many cartoons these days seems to be more towards a non-serious comedy style rather than the serious drama style. While there have been some serious action cartoons (TSSM, Wolverine and X-men, TMNT 2K3), a lot of the most recent offerings have skewed towards lighthearted romps that focus less on character development and plausible storylines and more on wackiness and humor. Batman: The Brave and the Bold is an excellent example of this - fun that it may be, it's almost uncomparable to B:TAS or even the more serious episodes of the Batman. And Marvel, of course, had to follow with a comedy mash-up of their own, in the form of Superhero Squad (although, from what I've read, it doesn't even compare to the Brave and the Bold quality wise).
I think the general reason for this is becase, by and large, the vast majority of people in America still see cartoons as primarily a comedy genere. When then think of the word "cartoon," they don't think of "animated stories," they think of "wacky and cartoonish." This is backed up by the fact that even the biggest and most sucessful action cartoons have generally paled in notability and sucess when compared to prime-time comedies like the Simpsons and anything by Seth Mcfarlane. Comic book cartoons have by and large always been a niche genere, just as comic books in general are a niche product.
Which is really twisted, IMO, as "mature" content in no way guarentees that a product is actually mature. You can load a movie or TV show with as much explict violence and images as possible, but in the end those things pale in comparison to characters and story. Sadly, much of the mainstream public doesn't seem to really get this. :shrug: