By extension, they are playing a role, and being mean to them is taken as being mean to the player.

Case in point, I was playing a Water Genasi from a little fishing village without much in the way of worldly knowledge. He encounters the party later in the campaign, and speaks to the monk. As a Half Elf, it has a different colouration to the rest of the party, being otherwise all human, while a new Water Genasi actually seems like something she should get along with. The Genasi's flaw was not being completely aware of social interactions and being very literal, despite his stupid high Charisma (Warlock, rolled 18, +2 at ECL4), but this would eventually peter out in the campaign as he became exposed to the ways of the world.

I decided to have a little fun when she asked 'Do you like the colour of my skin"; something she was evidently proud of as part ofnthe Half Elf culture; having the copper skin closest to Elven Ancestry being significant.

'No' replied the Genasi. From then on, the monk hated me thinking me racist, and had to explain that it was in reality me just literally saying I don't like the colour of the skin, but that she was a half elf doesn't concern him. He in reality doesn't know this.

Later joined by a Dragonborn, he also has fun when he mixes up casting a spell, and he asked in the Warlock knew what he was doing. "You all look the same anyway". Incredulity, but DM goes 'its canon'.