Kinda seems like there's some fabricated idea of people only liking 3rd parties because they're non-Nintendo, and disliking 1st parties because they're Nintendo, when it's actually pretty simple.
I can only speak from my own perspective, but I'll share it. I've said before that I can be steadily convinced of a character by seeing gameplay. I quickly shut up about Robin after seeing their gameplay for Smash Wii U. That was after previously saying, "No, it's gonna be Chrom!"
I've played at least one of each series with a playable character. I have an idea of what/who the characters are that folks want from Nintendo games. Bandanna Waddle Dee, Waluigi, a Gen 8 Pokemon, Zelda. To me, there really isn't anything inherently wrong with wanting a character from those series if that's what you're into.
However, with the way the DLC has been handled historically...it's kinda easy for me to understand why folks may not be especially excited considering the circumstances of that pass, and the potential for an actual pass like that to exist.
Mario has: 16 stages, 9 characters (though Yoshi, Wario, DK, Diddy, and K.Rool could be tangentially related, but I'd argue DK is a bit more removed imo), 106 music tracks, 13 items, 4 Assist Trophies (6 if you consider Klap Trap and Ashley)
Pokemon has: 7 stages, 10 characters, 33 Music tracks, 2 items (55 Pokeball Pokemon NPCs)
Zelda has: 9 stages, 6 characters, (45 music tracks) 6 items, 4 Assist Trophies
Kirby has: 5 stages, 3 characters, (38 music tracks), 4 Items, 2 Assist Trophies
Fire Emblem has: 3 stages,8 characters, 52 music tracks, 1 item, 3 Assist Trophies
That makes 37% of stages, 44% of characters, 28% of music, ~30% of items, and 22% of Assist Trophies. The numbers shoot up just a tad if you include Yoshi, Wario, and DK items. Nearly half the roster comes from these 5 series, over 1/3 of the stages come from these 5 series, over 1/4 of the nearly 1,000 tracks of music come from these 5 series, just under 1/3 of all items comes from these 5 series, and Assist Trophies are just under 1/4 of the total.
Mind you, there are 36 unique series with playable characters in Smash. 5 of them make up a significant portion of the content of the game. Clearly, some have more than others.
Here the thing though...the 3rd party characters that folks talk about. Let's think about some of them...we've got Dante, Crash Bandicoot, Travis Touchdown, 2B, Ace Attorney, or Tekken...just to name a few.
Devil May Cry: 0 Characters, 0 stages, 0 music tracks, 0 ATs, 0 items
NieR Automata: 0 Characters, 0 stages, 0 music tracks, 0 ATs, 0 items
Crash Bandicoot: 0 Characters, 0 stages, 0 music tracks, 0 ATs, 0 items
No More Heroes: 0 Characters, 0 stages, 0 music tracks, 0 ATs, 0 items
Ace Attorney: 0 Characters, 0 stages, 0 music tracks, 0 ATs, 0 items
Tekken: 0 Characters, 0 stages, 0 music tracks, 0 ATs, 0 items
You get the point.
Now, I understand Smash started as a Nintendo crossover. It still is a Nintendo crossover. However, the series itself has grown, and 3rd party characters are a major reason for that. Smash isn't what it is today without 3rd party companies and developers. Nintendo isn't what it is today with 3rd party companies and developers. Smash has become something that stokes the crossover dreams of fans, so it's no wonder fans will think, "Oh boy, I wonder which new series will cross over next!"
It's like getting an Ice Cream cone with 9 scoops on it, and while the next person says, "Excuse me sir, can I get 1 scoop?" and then the person with 9 scoops say, "But wait, what about mine?"
There's nothing wrong with wanting more, but understand that others may not be as open to it when their favorite/most desired has literally nothing in Smash. Nearly every major Nintendo protagonist supporting crew member, and villain is in Smash, so it's not all that surprising that protagonists of larger/popular non-Nintendo series are getting added over lower tier Nintendo characters. That's not something that I'm placing my bias in, it's just where we're at.
That was...a lot to type. Just some food for thought. Mario, Zelda, Pokemon...they are among my favorites. I grew up with them. They're part of who I am. But I've played a lot of games over the years, and at least for me...considering the amount of content we already have, I'd be much more excited to see what we get from something like Devil May Cry as I would be seeing 10 more Mario songs added on top of the 100 we already have, you know?