I was actually kind of disappointed by NSMBWii's multiplayer. It's pretty good in two player, but as you get above that people start running into each other when trying to perform basic jumps, or jumping into the bottom of someone else who jumped a second before, which leads to death, which leads to that weird pause on someone's death when everyone's controls stop taking any input, which often messes up the timing on people's jumps, which leads to more death. Add to that the whole turning into a bubble thing, and if people aren't really going out of their way to work together the game seems practically random with four players, and because it's such a finely tuned, skill-based platformer in single player, it makes you repeat sections way more often than LBP, which becomes annoying when you only died because you couldn't quite account timing-wise for the game freezing twice in a row when your teammates died while a monster was a step away from you and you were in mid-air.
That said, Mario clearly wins in single player. And I probably won't play a multiplayer game this gen I truly enjoy more than LBP, so even if mario multiplayer didn't seem oddly unpolished to me, I still probably wouldn't put it above LBP.