So your argument is what, exactly? That Cartman is written to be a likeable character; that he is supposed to be endearing? He is written as a bad person, one whom you aren't supposed to want to be like. So, when he does something racist, no matter how much shock value they put into it (hating jews for example), it is as a show for how not to be. Same for Rick from Rick and Morty, same for when a character on Family Guy makes a kike joke.