Wuhu Island is that place from Wii Sports Resort, Pilotwings Resort, Mario Kart 7, and Super Smash Bros for Wii U. I'm sure I forgot at least one of them. But whatever. Wuhu Island is another tour-type stage, like Skyloft and Delfino Plaza, visiting areas from Wii Sports Resort. You go to the beach, the arena place, the fountain, on a boat, and to other places that I'm too lazy to describe. This stage, unlike Skyloft and Delfino, actually has it's inhabitants there. The Miis do all sorts of shit, completely unfazed by your fight. Yes. I guess that you could say that Wuhu Island is the Mii's home stage or something, I dunno. One thing to note about this stage is that it's in the same location as Pilotwings, and the plane from that stage can even be seen in this one. Neat stuff.
Also, this is the last stage page made for the Wii U version (until Miiverse comes out). WOO HOO!