Simply have to weigh in on this... oh for... that was not an intentional pun.
My immediate reaction was disbelief and even seeing the final smash in GIF form I assumed it was an assist trophy, despite that making no logical sense. Because I really didn't want it to be true, but admittedly for fairly dumb reasons. There are a couple of facts that make this addition sensible.
Number one, she's the "main character" of one of the most popular new IPs by Nintendo, in fact one of the only new IPs by Nintendo in the last ten years. She's also a female and totally unsuspected by almost the literal entire internet. That is in a way impressive, though I can see how people may interpret Sakurai's comments as an attack on the core audience. For those who do, please respect that these were translated comments and just because he took the WTF factor into account, doesn't mean it was the number one reason.
Seeing the video and how unique she is sold me on the character better, she does seem like a good addition in terms of originality. There's no character remotely like that.
What also makes it a good pick is that it's clearly not a big slot that she takes up. The model is lower-res than others and the moveset is not flashy or complex in the slightest. By comparison, Villager plants trees and MegaMan has an arsenal of robot master weapons. Wii Fit Trainer has yoga and some very basic projectiles / special effects. Economically for Smash, as a couch game designer, it's probably not going to affect say, Ridley or K. Rool's chances.
In terms of the wider picture, I think this makes it clear that Sakurai is prepared to add literally any character if there is good enough reason to. I also really hope this is a sign we have quite a big roster, if Sakurai can throw a spot to a character like this. That is yet to be seen, but this is certainly an intriguing choice for the roster.
The question for me is really the roster, of course. I'd love if this character means we're getting a bigger roster than we thought. I really hope so!