In an interview with Wired, Mario creator Shigeru Miyamoto said that Super Mario Galaxy 2 will have a bare bones storyline. He mentioned that the original Galaxy had a bit more of a story than the typical Mario adventure, and that he believes Galaxy 2 should return to its roots. Story-wise.
His reasoning is that Mario games have never been deeply focused on intricate stories. The Mario universe is so rich with color and fun that a complicated sotry, in his opinion, can weigh down the fun.
Now, I don’t fully agree, but Shiggy seems to know exactly what he is doing with his series. Either way, I am sure that the game will be fun. Which is exactly what he is aiming for.