I think Smash might pass it for a short time, but then Mario Kart will retake it later on. Once Pokemon Gen 8 comes out, I think Mario Kart will be at around the same level or slightly above traditional mainline Pokemon sales. And unless Gen 8 is a groundbreaking, revolutionary leap for the franchise like Breath of the Wild was for Zelda, then Gen 8 will never catch it. I think Mario Kart 8 Deluxe will end up being the best selling Switch game when all is said and done.
Hardware Comparison Threads:
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020