I'd even say the numbers for 3DS are not bad, considering two things.
1)adoption for new hardwares in USA is much slower than elsewhere. Even DS with the Lite...it didn't peak in 2006. No, it peaked in 2009, with 3.31 millions sold in December. And it was still selling a massive load with the 3DS already launched ( pre-price cut ). Wii as well peaked in its third year on the market. And I'm talking about hardwares already at a mass price, since 360 and PS3 peaked much later than their debut, but also due to being so high in price at their launch. Numbers become bigger and bigger due to the lineup becoming bigger and better, and long sellers picking up forever, at least the Nintendo ones.
2)the actual economic situation in US seems pretty rough. And we saw it when almost all consoles decreased from November: the Black Friday deals attracted most of the Holiday buyers for how they were convenient. Only 2 consoles escaped from this: DS for... being DS, and 3DS, due to Mario Kart and massive advertising. And now, a sharp decline for every one.
About the lineup: well, NSMB is gigamassive, but it's true that so far it's one of the few huge relevant title for 3DS scheduled this year. The other one is Animal Crossing. But what else? Considering what said by Iwata, probably we'll have a pretty good lineup of first party at least for the last four months of the year. And maybe there's a third party title which could join them: Kingdom Hearts 3D, for having Disney in it. But the last one is a complete jump in the darkness...