As soon as the game is art, then you probably can publish it without doubt.
I've seen the insane journey to the mental hospital and the bloody blood everywhere, even more there were two guys who were completely naked (even their genitals can be shown). The game's name is of course, the Outlast. Made by Red Barrels Games, indie game developers of course, not big brand yet.
As you see the outlast's graphic, it's somehow realistic. But this game is still listed on Steam and popular anyway.
Even the game which has realistic graphic with blood, gore, nudity, strong language is prosperous in Steam.
Then your game, as you described it will be cartoon graphic. Then it seems ok to release it.
I know Steam accepts any genre (even they accepted Hatred).
By the way, the ones who rate the game is not developers. The developers need to submit their games to somewhere with fee and wait the rate.
I brought some informative article for you. http://www.indiedb.com/groups/indie-game-rating-system.
Its summary explains everything I guess.