Americans have animated some VERY fucked up shit, you just never really hear about these movies because they don't get pushed that hard. If you want some excellent tier animation, great stories, and fairly mature themes, watch some of the DC Original Animated Movies. Or the Marvel Animated Movies. Some of these like "Batman: Gotham Knight" (which takes place in Nolan's universe) or "Ironman: Rise of Technovore" are even "anime" because of their art style + they're made in Japan.
And I don't know what you're saying, American's have caused a lot of improvements in animation too. Hell, they started animation. Anime was the Japanese's response to America's animation. I'm not going to say one if better than the other, because they both have great content. The difference is, there's a lot more watered down cartoons for kids, so you may have to look a little bit harder for the good stuff. It's still there though, and there's quite an ample amount of it.
At the end of the day, even if I like American stuff a little bit more, my favorite story across any medium (Naruto) is an anime. They're both great, and anyone who says one or the other sucks is an idiot.