American cartoons were great at one point until they started out sourcing jobs to cheap studios that sometimes produced very bad animations, the same happened to Japanese cartoons for awhile as well. Then came the current style of CGI animated cartoons that have more stability but you can always tell if it was out sourced at the good companies or the bad ones even today, but as far as making more detailed animation goes, unless it's funded to the tee, Japanese has the better animation on average because their art is more detailed, Japanese animation stories can be cliche but not as cliche as most American ones, and their VOs are better dramatized in general.
The problem with most American animations are that they are aimed more towards children, whereas Japanese animations cover all grounds, so there is a huge variety difference to start with and that's why American animation generally can't compete. The last great animation from America that I actually saw in the movies is still Titan A.E. and that came out 13 years ago! I have not seen anything close to that level afterwards, not even the new Heavy Metal from 2000 was that good, in fact, the new Heavy Metal sucked giant balls compared to the 1981 original.
