It's tempting, but there are so many issues with that.
For one, picking the fighters. A usual max amount of characters in a fighter is 30, Cartoon Network has aired nearly hundreds of cartoons, and plenty of them popular, even if off air, to stuff in, but at the same time too many. Fans would argue selections.
Another thing would be target audience. CN catches the attention of kids and yes, even us in our 20's and up, but well, they lost a lot of popularity recently, so the audience is mainly kids. You have a better chance marketing it on the Wii, since they also seem to do a lot with CN, that or Sony. The 360, despite having kids who made their parents buy the console, is mainly targeted to older audiences.
There are other things to discuss as well, but I just wanted to lay out my main thoughts. I myself would love to see a fighter for CN characters, but it's just something that may not happen.