Smash is naturally a Nintendo game. But it's easy to see that in the last years Smash has received a good amount of 3rd party characters. Of course there's gonna be more Nintendo characters because 1. it's a Nintendo game and 2. Smash didn't see it's first third party fighter until Brawl, the third installment of the series. Of course there's gonna be an "unfair" ratio.
But I think it's pretty clear that Nintendo and/or Sakurai had been very happy with third party inclusions for one reason or another, what would explain the rise in not only 3rd party characters but also 3rd party content in general.
I mean, Smash was already a gaming crossover before Ultimate or Smash 4 DLC's, because Nintendo it's an important part of this industry. A fighting game featuring multiple Nintendo IP's is a Nintendo focused crossover, obviously, but that doesn't mean it isn't a gaming crossover or a celebration of gaming. This and the fact that Smash has been very friendly and open lately to 3rd party content in general aren't mutually exclusive.
Honestly, I think everyone gains from this. People who wants more first parties will still receive first party content, and people who maybe prefers third party characters knows that Smash is gonna still seeing new third party content in the future.
Imagine if by the end of the second fighter pass, the very last third party character introduced to Smash is Terry Bogard.
I don't see anything wrong with this.