Now I know what you are thinking, how could New Super Mario Bros 2 affect the Wii U when it is a 3DS game. In reality everything in the entertainment business fights with each other, including one's own products againts itself. I know you have heard that Smartphone has eaten into handheld sales but it has also eaten as well into the mp3 players. I myself don't use my old classic iPod because now I have my iPhone to carry my music and more.
Back on topic, New Super Mario Bros 2 could actually affected the bad sales of Wii U. It has to do with series fatigue, New Super Mario Bros 2 was released to close to the release of New Super Mario Bros U. New Super Mario Bros and New Super Mario Bros Wii where released about 3 years apart but NSMB2 ( abbreviations from here on out) and NSMBU was just a mere 3 months in America. I am huge Mario fan, but even I know Nintendo definitely tried to milk NSMB series onto the 3DS and Wii U. If they had space them out each would have a bigger effect.
Nintendo obviously had planned to use NSMBU and NintendoLand as evergreen titles for the Wii U to continue selling. I remember that Reggie said that unlike the 3DS the Wii U now has launch Mario Title which "all" of were so eager to getting.
Remember Guitar Hero, well they use to release so many editions so close to each other and now the brand is dead with no hope of revival soon. I also here gamers complain of Call of Duty yearly releases. Nintendo should have seen that not even 2D Mario can escape from series exhuastion.
And don't get me wrong NSMBU is a great game. In fact NSMBU is arguably the best game of the whole NSMB series, but for some reason I haven't willing to come back to play it over again as frequent as NSMB and NSMBW.
What would I have suggest Nintedo to do instead? Well I think had they released NSMBU in January or Februrary would have been a great timing to boost Wii U. Obviously one wouldn't be able to see that the Wii U was going to drop so low and quick. Or maybe it is just the price or the lack of Wii U Gamepad not being as innovative as the Wii Remote.