Yeah...you're not the first person to troll the board and accuse the game of being kiddy or childish, and you probably won't be the last. Doesn't change the fact that this joke has become incredibly unfunny and sad.
It looks like what it is; a Ghibli movie in game form. Do you think you'd look silly watching Spirited Away, or Howl's Moving Castle, as an adult? As someone who just turned 22 I can say for sure that I wouldn't, because they're really good movies with some amazing artwork.
A good game is a good game, regardless of its tone. All that matters is quality and consistency, and Ni no Kuni has both in spades. So, in short, no, not at all.