Mentally handicapped people used to be comedy fodder - people who weren’t just foolish like Homer Simpson, but rather people who were clearly disabled. For example, in the radio show Duffy’s Tavern there’s a character described by the lead character, this way
a subnormal chowder head . . . a dope . . . a low-grade moron.
You have to forgive him. When he was born, the baby doctor was a little nearsighted and Finnegan got slapped on the head.
That has dropped off considerably.