Peter: *curses*
Sam: Wow, do you kiss your mom with that mouth?
Peter: Jokes on you, I don't have a mom.
Tony: *having a heart attack* AFSJDHFKJJD Peter, we talked about this!!!
Listen up, you smooth-brain! The joke is that Peter's comeback is savage because he's an orphan. Sam asks if he kisses his mom with his dirty mouth, and Peter hits him with the "I don't have a mom" card, which is dark humor at its finest. You probably ask people about their day, don't you?