Someone makes a bit of a bigoted character, which isn't unusual and not necessarily bad. It works when the person isn't a racist because often the joke or point of that character is that being racist is bad and that they're going to grow, or flanderize into an It's Always Sunny character.

But when a racist does it, you start to see real world bigotry inserted into the game and it gets awkward. It's not usually overt but you hear them say something and you feel the tonal shift. I used to DM and I had player characters that were heavily racist, but it was obvious satire and they knew & expected that there would be consequences for their character in the context of the game.