I always hate when people seem to come across a fact like this and think they've stumbled upon the holy grail of secrets; it's kind of a ***** slap to the industry I admire and actually have family who works in. My sentiments are that in a medium like Cartoons and Comics if it's drawn and written by a mature adult with passion for their work then it's a Mature piece of work regardless of presentation. That's of course taking the word mature by it's actually meaning instead of it's perceived one.
In it's perceived meaning, maturity is reserved for only adult subjects like harsh violence, sexual tones, and in a lot of cases depression.
In it's actual meaningMaturity is a psychological term used to indicate how a person responds to the circumstances or environment in an appropriate and adaptive manner. (Wikipedia), Mature content is smart content. Content that doesn't attempt to belittle it's audience or act to the lowest common denominator just because it's easy.
In this vain, two shows that can be considered mature in it's perceived meaning; Futurama and Family Guy; in the actual meaning I would separate into Futurama being Mature and Family Guy being Immature.
With this I can say that some of the most mature animation I've seen on TV and Film have been; Animainiacs, Batman: TAS, My Little Pony: FiM, Avatar the Last Airbender, any Pixar Film, and any Miyazaki film.
If you want to break it down further you can call out the categories of "Kid Show," Family Show," and "Adult Show." But in all honesty those labels have nothing to do with Maturity. After all, Sesame Street fits perfect under the qualifier of "Kid Show," and they never come off as immature to me.