Explanation for Po's behavior in show.

I think the in-universe explanation for Po's poor behavior in the show is that he literally became from just the random waiter at his (adopted) dad's to a widely admired hero in just a week. I think he just had a hard time adjusting quite a radical change of lifestyle. And all that attention from being the Dragon Warrior probably took quite the toll on Po's personality. By the time of Kung Fu Panda 2, Po probably learned how to handle his new found fame a bit better.