Quite frankly, I really enjoyed the game, and thought it was better than the first. Besides, if it ain't broke, don't fix it like they say.
Also, Mario games are mostly focused on gaming than the plot, and IMO, gameplay is the utmost important thing in games while plot is of course 2nd. SMG2 was fun to play and challenging esp. when collecting the green stars where it gets pretty frustrating. If Mario was focused on plot, we might get crappy stories like with some of the Sonic games out there, or worst, might get something similar to the Super Mario Bros Movie.