Nintendo's own forecast is 1.2 million more Wii-U's sold so unless they are planning a major price cut offcourse Mario Kart is going to boost sales.
It will give it a nice bump and will have nice legs, probably boosting Wii-U about 10k on average the rest of the summer.
Though i do fear that Smash Brothers will do hardly anything for hardware, because most people waiting for that game are also waiting for MK8.