So here's the thing, while I think boycotting Smash Bros Ultimate for this reason isn't logical, I also think they
somewhat have a point.
Most Nintendo games have generically light-skinned protagonists. This is because Japan is mostly homogenous and most Japanese people are light-skinned. So of course there are few black characters in Smash Ultimate. Having said that, Nintendo has made strides in improving this recently, with players being able to change their player character's skintone in Splatoon and Pokemon games. But there still hasn't been a major Nintendo character who is not light-skinned.
If Nintendo were an American company I'd say this was pretty uncool, but being Japanese, it's a different culture and it isn't really fair to judge them in the same way. Somewhat related, I would recommend people check out
this YouTube video.