It varies from cartoon to cartoon for both western and japanese cartoons. the difference is that Japan produces far more animation aimed at adults, and some of the ones aimed at kids are surisingly mature (look a Studio Gihbli's films, or even Astroboy), which is somthing you don't get in western animation so much (Somone mentioned the Batman cartoon series that aired in the 90's- that being the only westrn cartoon that follows the "don't talk down to kids approach" I can think of offhand).
An example for both a Japanese and a Western cartoon that is mature (meaning both aimed at adults and not crass/gore obessesed) would be Monster (an anime adapted from the manga by Naoki Urasawa) and Persepolis (a french film adapted from an autobiographical graphic novel by Marjane Satrapi), both japanese and western respectively.
An example for both a Japanese and a Western cartoon that is mature (meaning both aimed at adults and not crass/gore obessesed) would be Monster (an anime adapted from the manga by Naoki Urasawa) and Persepolis (a french film adapted from an autobiographical graphic novel by Marjane Satrapi), both japanese and western respectively.