Mature as in "blood tits and drugs and swears", or mature like "you need to be this old and this smart to understand the plot"?
Most "you must think to get it" animated films came out in the recent 10 years. Have you (or anyone) seen that brazillian movie about an immortan guy who keeps living through the country's civil wars? I haven't heard anything past the trailer that came out.
I'm really not sure how "mature" this one counts as, but I'm pretty certain most American parents wouldn't let their kids see it.
I think the best one is Fritz the cat.
Fantastic Planet is one obviously, I'm still pissed we don't have a blu ray for it in this region.