How can I politely ask people to stop calling me ugly?

Just shake it off. Looks aren't something to cry about, cause you're going to live with how you are the rest of your life. It's not going to change. Just get used to it and learn how to cope with it. Don't let people know that it bothers you, because they'll take advantage of that and just make fun of you. Life is better off when you don't worry about how good you look.