I wouldn't make the 2D Mario comparison against Mario Kart. The market clearly accepted Mario Kart 8D as THE Mario Kart of the Switch. If they hadn't I'm sure we would have seen MK9 on the Switch, but since consumers accepted it (and therefore Nintendo too has accepted it) as the Switch's one and only Mario Kart and it is selling as though it is a brand new Mario Kart.
NSMBUD is not the same. It's a port of a last gen game at full price that launched in January as just a sort of post-holiday filler, and for a 2D Mario it is selling as such. It even has the Wii U branding in its name, unlike Mario Kart. I think everybody expects Nintendo to make a 2D Mario for the Switch, and it would be shocking if they didn't, which will trounce NSMBUD sales. It's basically a filler Mario game until they release a Switch 2D Mario, while MK8D is the Mario Kart for Switch. Still though, the last gen port of 2D Mario will sell over 10 million, which should suggest that a new game would likely sell over 20 million, putting in on par with the top selling portable 2D Mario's of the past.