I have a number of problems with this cover. Overall, it lacks the Nintendo style. It consists of only a handful of characters revealed so far. That fact alone does not prove it to be fake, but the choice of characters seems random/inconsistent. The background is incredibly plain: it is a bright blue flash. Where is the detailed battleground with characters facing off against one another? Also, look closely at the characters. They have a rather odd style to them. I know several good artists who could probably pull something like this off. Odds are that the final box art will be something from Nintendo that no one could replicate on their own.
Basically, I would be rather disappointed if this were the final cover. It really is a high quality piece of work, and props should go to whoever created it. Sadly, it is not Nintendo caliber. (And the adding of the WiFi symbol later is a blatant signal that there is no remorse in further modifying the picture. Obviously, the final art has yet to be revealed.)