For example, many phrases from Black American culture have been appropriated by white Americans
Memes are a great way to get an insight into the culture of people but what is important is that it is those people making memes about themselves. Looking at memes made by black people that are poking fun or making references in black culture can be a great way to better understand black culture. What is problematic is white people making memes about black culture, because that is not coming from a place of deep understanding and critique but from prejudice. Its kinda like doing accents, if you as a white person do a impression of a marginalized group it better be really damn good or its gonna be racist. And if you can do a really good impression its likely that you are coming from a place of deep understanding of the culture because you have spent a lot of time with its people or something else.