He's not a bad choice, as he represents one of Bowser's main henchmen, which fits his role perfectly in the Solo Mode. However, I think Mario Party should contain more good characters, participating in random and fun games, all playing childishly, so I don't really think Kamek is very childish.
And actually, MP9's storyline is so far the most ideal in introducing Rosalina as a playable character, what with the Mini Stars (from outer space) being stolen. Rosalina could've been perfectly incorporated into it all, with Toadette randomly appearing as her assistant.