It's called Mike Cover for Wii U. And yes, it really looks like a toilet plunger.
The Wii U-branded, officially licensed peripheral is designed to be used with Wii Karaoke U, the Wii U karaoke game. It's basically a hunk of plastic you put over a microphone, but it apparently reduces noise, so people can sing karaoke without disturbing their neighbors. In cramped Japanese cities, that might be necessary!
So, for example, regular conversation is 60 decibels, and the mic cover can turn 80 decibel singing into 60 decibel singing. The catch, though, is that you have to sing into a contraption that looks like a plunger.
This karaoke sound reducer isn't new and has existed in Japan for a while—heck, one was released one for the Wii back in 2009. Though, I've never seen this used or known anyone to own one. And if you are really concerned about disturbing your neighbors, you know what's the best thing to do? Go to a karaoke parlor.