I find it funny that 1st and 2nd party picks are somehow seen as advertisements or shill picks because they come from...Nintendo. Like, what??? That makes no sense in the dlc smash community. Even calling Byleth that is a bit of a stretch because Three Houses was likely already considered to have some form of representation.
That being said, the idea that a Nintendo pick is purely advertisement based is stupid. Corrin was added because Fire Emblem's popularity has skyrocketed after Awakening and they felt the new game would probably have similar success, even if it didn't. Makes me wonder how people would act if a Sword and Shield rep got in smash, considering the gigantic divide in community it brought with it.
One thing to keep in mind is that when Corrin was announced, Fire Emblem Fates hadn't been released anywhere in the world but Japan at that point. (plus,
Corrin was explicitly included to promote an upcoming game)
As for Byleth, the whole "advertising" aspect seems to come from the fact that Rex (whose game was announced January 2017 and released December 2017) was too late to be included in Smash, while Byleth (whose game was announced (as a placeholder, meaning that they likely weren't far in development) January 2017, revealed (given an actual title and details) on June 2018, and released July 2019) made it in, which (coupled with Roy and Corrin) makes it seem like they're once again using Smash to promote Fire Emblem.
With Min Min, it seems like a stretch to call her a promotional pick, but the lead up to her announcement likely got more people talking about ARMS than the game itself ever did, which seems like it could have been their intent.
While characters like Bandanna Dee, Dixie Kong, etc. likely wouldn't be dismissed as a promotional pick (they've got a long history behind them rather than being tied to one specific game, there's no games that they'd be promoting, etc.), Nintendo seems to have a track record when it comes to new first party DLC characters. (seeing as three of the four newcomers could be arguably seen as promoting something, and the fourth is Piranha Plant)