Who says Munchlax is the DP mascot, please stop making things up...he's there to HYPE the game as a new Pokemon, no one said that he's the mascot yet. You could say Marill was the mascot for G/S has he was featured in Orange Islands and was "plastered" all over Japan...look what happened.
Actually, Munchlax I believe was stated to be "the next Pikachu" by GameFreak.
I don't actually have the article, but I do remember hearing it. Back when we first saw it.