I love American cartoons, and I don't see how you guys can obviously say that they aren't as good as Japanese cartoons. It's not even the same ballpark. Japan is very different and they have their own separate cartoons for young kids and for teens or even adults. But America, mostly everything is family orientated .. so of course it's not going to be "as good" or "as action packed" as what you're comparing it to... you being teens.
No they're not getting dumber, they just need some new ideas. Networks like Cartoon Network and Fox really really like to drain as much money out of something that becomes a huge hit.. so they keep making more until people don't watch anymore. After all that, they finally take a risk with some new show that you love at first and then eventually hate a few years later after seeing it for so long.