As someone who passionately disagrees with Nintendo's stance on emulation and ROM distribution, I don't think there is anything wrong with buying their games.
The decision to chase down ROM sites is a Nintendo corporate decision, not one made by developers (which would be Sakurai in the case of Smash Ultimate). And while the corporate bigwigs are getting a cut of that purchase, I do still believe developers deserve a return for their work, especially if it's a good game.
It also comes down to the fact that they're releasing games you want to play. You don't have to do everything in life in the name of something bigger than you. I don't even think boycotting them would change their stance on this, at least not boycotting their games in general. I think nothing short of a change to the laws regarding piracy would change Nintendo's stance, and even THAT is debatable, as Nintendo has been known to break laws for as long as possible in order to maintain their own anti-consumer policies in the name of money, as are a lot of other video game corporations. The video game industry is still super archaic like that.
So yeah TL;DR you're allowed to buy what you want.
Maybe this tweet I made yesterday is relevant now.
There was a problem fetching the tweet