Context and Intent matter for racism, as much as they matter for everything else. If the context isn't racist and the intent behind it isn't racist, it's not racist.
This is not subjective.
Cosplay without ill intent isn't racist.
Painting your face isn't racist.
The color black isn't racist.
How do you logically arrive at "this is racism"?
Now, you can personally be offended by whatever you want. But it's just your subjective opinion. And since it is subjective and based on feelings instead of facts and logic, it doesn't need to matter to anyone else.
However, that doesn't mean I think it's fine to do this in the US. I see this like the nazi symbolism in germany. It's alright for a nation to ostracize/outlaw certain practices/conduct based on their historical significance. If the US, as a society, decide this shouldn't be done out of respect for their fellow citizens or whatever, that's fine. It slightly changes the context.
But you shouldn't force that stuff on the rest of world. We had
star singers in my country for quite some time and I don't see at all how that could possibly be called racism. It's just that we did not have enough black children to fill that role, since the refugee crisis it's more often filled by black children.
It's fine for Twitch to ban her, it's US based.. but objectively, she didn't do anything racist.
What's next? Male cosplayer dresses up as female character, LGBT+ and feminists call for his head because a male dressing like a female, that's totally sexism, right?