Was Wii Fit a big system seller? Not doubting that comments reality, just asking a serious question. I know Wii Sports was. I didn't know Wii Fit was.
Personally I don't think either would have the impact of Wii Sports on Wii simply because it was something new and had a certain charm that is very hard for developers to recreate. Also, WiiU is not popular. And you have the Kinect now.
Wii Fit was probably one of, if not the most, system selling game ever. I'd say it's largely responsible for the Wii selling as much as it did in 2008.
In regards to the OP, it's really too hard to say what kind of impact it would have on the WiiU. The fitness game market has completely changed since Wii Fit's hayday.