I originally replied to your comment about MK8 would have shown people about "new experiences".
If now you're talking about graphics purely, this wouldnt have done much either, because the problem isn't about how much WiiU game look better than Wii games, but about how they don't look like a big step up from PS3/360 titles. The former would only matter to you if, as I mentioned, you've been completely oblivious to anything else than the Wii.
When I said "experiences", what I was really referring to was something new. I apologize for the impreciseness.
Mario Kart 8 looks significantly better and, for lack of a better term, "newer", than any other Nintendo title. Hence, in building up hype for launch and beyond, I believe MK8 would have done a much better job than NSMB U which for the most part looked like NSMB Wii in HD.
Part of the problem, as evidenced from sales so far, has been a lack of adoption from the hardcore Nintendo loyalist- those customers I suspect would have been more excited about the system and its future if their first look of the system was MK8 and not 2D Mario.