That is an odd comment. Excitebots Trick Racing, which is pretty graphically intense, I think, manages really fine online play, yet NSMB Wii taxes the Wii so much that it can't do it? It doesn't make any sense.
It's a shame there's no online, but truthfully isn't gameplay like this a lot better when your friends are in the same room as you? So I'm not going to sweat it, but that doesn't mean I don't wish they'd included it. If they had, I think it would have also required Wii Speak support, as otherwise I don't see where it would be all that fun.