They are mad because DBZ is probably THE most unifying element between all races, and successfully disproves every single narrative these people put forth.

Because every black, Mexican, Asian and white boy has a strong opinion on Goku/Vegeta and will absolutely embrace each other as brothers over it. Given that, they must bring their racial separatism into the fandom.

Hero Academia isn't by him, he's known for Dragon ball and basically setting the foundation and being an inspiration for tons and tons of mangaka. This person mentioned here is pissed because of Mr Popo who isn't even meant to be a black guy but doesn't change the fact that these people feel the need to crap on someone's grave.

Because he just died which means they can crap on him without fear of retaliation from him, for obvious reasons, and he's obviously a big new item right now so they can yet again parasitise off of another by using the news cycle interest to inject their own retarded words into things again.

Because he has cultural power which threatens their own, even in death.

because he's popular and what he created was beloved and far beyond their ability to ever pervert it for the cause.