A pizza boy is generally the outcast in the group. Onlookers wonder what said person is doing interacting with a group of people who are obviously cooler than the pizza boy, and conclude that the only possible explanation is that said person must be delivering a pizza. An insult.
As taken from the "Mad Real World" skit on Chappelle's Show (Chad)
...John shows up at a party...
Steve: "Hey, who ordered the pizza; The pizza boy is here!"