IDK, Yokai Watch was big a few years ago but it's fallen hard. I wouldn't expect to see it pop up in smash, I don't think it's a game series many people actually care about, it was more of a fad.
Japanese Sales (Combined Versions):
Yokai Watch 1 = 1.3m
The series started, peaked and died with the 3DS.
Yeah, it would have made more sense if it came out on Smash Wii U/3DS. Still, with 4++ being on the Switch, I'd say it has a chance to be in one of the fighter packs.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/