I'm not entirely opposed to Mimikyu, I just don't find it extremely likely and I think most of the reasons for it are ultimately people trying to find reasons that set it apart from the crowd other than popularity. Fundamentally, Mimikyu is still a Team Rocket Pokemon, its kinda a bit player that occasionally gets an episode in the limelight. Generally, you can tell when a Pokemon was intented to be "pushed" as they get a lot of time in the spotlight even among the larger cast. Rowlet and Lycanroc are good examples of this, the former being the frequent comic relief of the series and the latter being the ace of Ash's team.
Also regarding the "Wario" argument, I don't know if its a good justification for Smash. We only really have two "dark versions" of a character in Smash, Wario and Dark Pit. Wario's role as a character (at least in Smash) isn't really based around his rivalry with Mario, it's more based around his appearance as a greedy, gross businessman in Warioware. Dark Pit more or less got in on luck, being an easy upgrade of a costume.
Mimikyu is popular, there's no debating that. However, its popularity grew out of an organic attachment people had to the character rather than a marketing push. Recent marketing has featured Mimikyu in response to this (stuff like being a DLC assist in Pokken, getting its own Z move, and Totem Mimikyu being available in both US/UM). However, this is lack of an initial push is what I believe would cause Sakurai to pass him up in the initial roster.
I'd say Mimikyu is probably the fourth most likely Pokemon to be added from gen 7, after Rowlet, Lycanroc, and Decidueye.