Oh I know why it is. Nintendo is nothing if not completely shameless when it comes to obtaining money, whatever the cheapest way possible to said money may be.
And it's not even the idea of Mario Mii's itself. Mario is always whored, and low-effort shit is par the course for launch windows. This is easy shit that Nintendo can put ten guys on and have the entire Nintendo community slogging it off and calling it a masterpiece for the cost of a dozen chicken nuggets. They know how to manipulate their fans. Chris Kohler is already writing three articles about the groundbreaking nature of this paradigm shift.
But man, it's bad enough I had to spend an entire generation looking at the hideous Mii's. Now they will follow for another generation, with every developer painfully shoehorning them in as that 'extra character' instead of, I don't know, designing an extra character with some real personality. I mean, shit, Mii's are so fucking lazy that all they really are is a head, not that this is particularly enlightening commentary. You're just really designing your head, because the body is mass producted two color shite that only Nintendo could love. Nintendo doesn't just cut corners in hardware, they cut corners in art design too. I kinda feel that when I see a Mii, it's also code for "this game is almost certain to be a shallow mini-game compilation of some sort mass produced for the casualtard set." Naturally there are exceptions, but not many.