Yeah, it's hard to say. For the original adult stuff on Adult Swim, there was a Aqua Teen Hunger Force movie, and there was a Metalocalypse music tour; there are devoted fanbases for each. But I don't know how much of the general public knows about it. (The 2007 Aqua Teen promotion incident in Boston suggests not a lot.) There's also a pretty devoted fanbase to anime, but I imagine the same applies in that the general public (particularly older people) are not quite as aware. Things are more fractured these days.
You're right that the "most mainstream" in animated television is shows marketed for teens that have crossover audience appeal. Kim Possible, My Little Pony: Friendship Is Magic, and Adventure Time in particular are fairly big from my perspective (there's a few others in that list that are arguably just as big, but my "first impression" is these three stood out). None of these are The Simpsons level big, but it's the best you can hope for in this day and age on TV. "Mainstream" is sort of becoming a dated concept.
The most recognizable animation lies in film at the moment. It's easier for me to picture a Toy Story or Cars or Monsters pinball for instance, the appeal of many of the Pixar franchises is very broad.
