If Smash and MK are not in the same genre, then why is OP even comparing their sales in the first place?
What's next? "Do you think MK will outsell Pokemon? Not that they have anything to do with each other, but I mean, just wondering anyway!"
Anyway, to answer the question, no. I don't think MK, despite being on many platforms, will never outsell SSBU, despite it being limited to one system.
But I mean, if you want to disqualify Smash based on some arbitrarily chosen rules of what constitutes a "real" fighting game so you can sleep better at night, I mean, go ahead.