I probably mentioned this other times, though I'm not fully against non-game characters.
It depends on the context though.
If a character joins Smash as part of a game, with the moveset based on that specific game, then I think i'll accept it... if a character instead joins just as a pop icon then it wouldn't make sense.
For example, I would accept a Mikey Mouse based on Epic Mikey or Kingdom Hearts, but I would be against a Mikey Mouse based on the general Disney franchise. If he has to join, it must be because of a game, he must have gameplay as source material and reference gameplay in his moveset.
The same applies for any other character in a similar situation (including manga/anime characters).
I would accept chracters from non-games when the non-game is an interactive software/media. For example, even if Miku Hatsune didn't originate in a videogame, I think she would somehow fit in a way.... at a certain point, the barrier between games and non-games blurs so much that it's hard to distinguish between the two categories.
Is Nikki really a videogame character...? She did some cameo in some actual games, but she's primarily the mascotte of a software... it's a software designed like a game, but it's still a messaging software. Is Mario Paint really a game? etc.
So, I think that there are some specific situations when non-game characters can fit Smash in some way, though I'm not a supporter of the idea of getting them in Smash, I'm just open to accept it if they will ever decide to open the doors.