Fuck that's a shitty opinion, like just on so many levels. Firstly, it's just as racist than if a person ran around squinting their eyes and putting on a fake accent. Just in general painting your face black, to portray someone who is a different race to you is such an insensitive thing.

I know you don't care, because you're probably not black but it's not a good feeling looking at someone making a joke out of your skin colour, something you were born with.

He is doing it as a joke, and the thought process was probably something like "let's paint my face black, that'll be funny"

And before you retort "they were just accurately doing Finn" that's a lie, because the whole costume is there to make someone laugh, not for accurate representation. So yeah, chances are they are either insensitive culturally or they are uneducated.