Mario Bros franchise is the most iconic thing Nintendo has. Mario Kart is of course a part of the Mario bros franchise.
Comparing the two series: SMB vs MK, well MK is a multiplayer game so pretty naturally it's gonna generally be more popular. It's the most popular and casual multiplayer series Nintendo has so it makes sense it sells more than Mario Bros. But Mario Bros is still the "main" series of games Nintendo has. Also Mario Kart is very similar from game to game, so it's hard to get nearly as excited as a new MK game as people get about a new Mario Bros game.
So while MK outsells Mario Bros, I would not in any way whatsoever say it has "outclassed" Mario Bros.
Mario Bros and Zelda are the two major series for Nintendo. After that comes Smash, MK, Pokemon, and well with how popular Animal Crossing was this gen maybe AC is joining that group as well now.
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