I think people get the wrong idea about reception toward first parties. I'm definitely not gonna deny that they get more heat now than they would have on the base roster... but hell, if Ridley or K. Rool were DLC instead they would still be beloved like they are now.
The reason people are adverse toward first party DLC is because of the precedent that has been set. Byleth, Min Min and Pythra are all characters from the last five years that debuted on Switch. Prior to that, there was Corrin who needs no introduction. This has no bearing on what I think of these characters mind you, but it's easy to see why there's some aversion toward the idea when Nintendo only seems willing to give their own blood the time of day for DLC when it's to promote a recent title / correlates with something new on current hardware.
Same goes for why say, Pokemon and Fire Emblem are met with groans but people are still open to Mario characters. Mario characters get added because they're popular and consistently relevant. Pokemon and FE only get characters when there's something new to promote. If we started getting some random Gen 3 or Gen 5 Pokemon or something, I think people would receive that very very different than they would say, Cinderace.
If we got Dixie Kong or Waluigi as DLC, or a new franchise like Golden Sun or Rhythm Heaven, or a new Zelda character... I am very confident that this would go over a lot better than what we've seen thus far. Granted Pyra and Mythra aren't even maligned characters, they were received fairly well given the unfortunate fate of XB2 on the base roster.