It's actually performing decently when you consider the shitty economy, higher than usual price for a Nintendo console, and the complete lack of compelling games.
NSMB U is the reason it's even selling anything at this point. You can say Monster Hunter and Dragon Quest, but those are really only big in Japan, and DQ is on a massive decline, mostly thanks to going MMO.
Once what I consider to be the Holy Triforce is released - Wii Fit, Smash Bros, and Mario Kart, you're going to see Wii U really pick up. Still won't touch Wii like numbers, but will be more like PS3 numbers, maybe even better. Most people think Wii U is done, when in fact it hasn't even begun.