I take each 3rd party individually.
I did used to be pretty purist and say nobody but Sonic and frinds allowed, but times change. Now both of my most wanted are third party, and I don’t really have enough characters I want to fill out a top ten.
In Brawl, I wad against Snake since I didn’t know him at all. His moveset was pretty fun though, so I’m not at all opposed now. Sonic is a perfect fit though, since being thr chief rival is also a major part of Nintendo’s history, and now they’re crossing over more. It’s a pity that we still only have Sonic - we should undoubtedly have Tails by this point, and Robotnik should at least be a boss if not playable.
For Sm4sh, even though I don’t really know Megaman, I’m really happy with him. He has a fun moveset and great music. Pac-Man is also a good choice even though I’m not a huge fan personally (and he brought the wrong stage back). Ryu I’m also okay with even though I don’t really know Street Fighter, I do still know the character. Cloud also is a character that everybody and their mother recognizes, even though super anime-esque RPGs are not at all my thing.
After that though, they’re getting more obscure. I’m not as anti as I was, but I would prefer some characters I recognize. I’ve never played Bayonetta, but her potential is interesting to say the least. Still, I have zero connection to her. Same for Ken, who I had actually expected to be an Alph-style alt to Ryu back in 4 (and Guile got the assist over Chun-Li?). Ditto Simon and Richter as well. And with Joker, it’s yet another Cloud for me - an incredibly anime-style JRPG protagonist exclusive to a system I don’t have but without the iconicness.
I’m definitely okay with getting third-parties in Smash now. My only thing is that I don’t play many different series, so a lot of the names thrown out are literally whos to me, so there’s no hypeworthy connection to make. But again, I could say the same about Nintendo characters as well - I always thought Dark Pit was my most disliked character, but that was when I completely forgot Shulk existed. And all of that goes back to my indifference toward that game and art style.
I hope we get a nice mix of DLC though. I’d hate it to be four anime JRPG characters and Steve from Minecraft.
Tldr:
I think the issue a lot of people have in general is that we expect Nintendo fans to have a relationship with most Nintendo characters. With third parties, there’s a greater chance that some people will love the addiditon. And some will have no clue who they are and are not interested in the games artstyle/theme/etc. And people naturally only like the characters they care about.