In particular in the west, 3DS probably constitutes the general Nintendo fanbase that's still left.
Problem with the WiiU is, it doesn't even cater to standard Nintendo fans. The library is completely unexciting to anyone who already had an original Wii or 3DS, there's barely any unique or ambitious content, plus for the first time literally no 3rd party support. And it's expensive, because of a gimmick they've clearly put no more than two minutes of thought into.
In fact, the two WiiU titles in the OP are the only relative success stories of the console right now because they are the only ones that aren't the most generic rehashes from the Wii/3DS libraries (plus not niche like Bayonetta), but clearly experiences you can only get on WiiU - even if it only comes from great new graphics in MK8, but that's already more than what most WiiU content offered up until now. So, a console with only 2 titles to write home about after two and a half years really isn't showing how small the Nintendo audience is, but just that WiiU is a bad machine that already sold more than it ever deserved.