It's a simple schoolyard insult.
For eons, people — often men — hurled "mama's boy" at each other as an emasculating put-down. To be called the son of a mother suggested an essential unmanliness. "Mama's boys" were comically inept, even pathological, in movies and television shows ranging from the pathetic Buster Bluth in Arrested Development to The Waterboy's Bobby Boucher to Norman Bates in Psycho.
