I recently looked at the plot summary at Wikipedia, and I instantly said, "Oh, I get what they're trying to do." These were basically trying to be "smart" with its ending. Yeah, I hate films that try too hard at being clever than they actually are.
Oh, and as for adult cartoons, look I get why animation wants to look mature, but animated films need to think what they will lose once they go through this step. It reminds me of "The Little Prince," the problem isn't growing up, it's forgetting. As much as I want animated films to be vindicated as works of art that are just as respected as they should be, I don't want animated films to forget the fantasy, the wonder, the enchantment of what an animated film can be. I want an animated film to win a best picture, but I don't want films to be just as packaged, processed, consumed, and forgotten as the food or jokes from this movie.