This is a phrase which is used in order to show the act of putting someone down. It's like when someone says something extremely dumb, leaving himself completely open to insult, and then another person says something which brings shame and utter humiliation
to the person who just said the dumb thing. A third person is then introduced, being responsible to yell the insulting phrase in order to make the first person feel even more stupid.
person 1: Did you see that car? It was ugly!
person 2: Your face is ugly!
person 3: YOU JUST GOT MOPPED!