Once upon a time, I thought of Jughead's cap for no particular reason, and wondered if anyone outside a Hollywood soundstage wore one. I didn't know it was called a whoopee cap until I read this thread, but I learned it was, indeed, real. Egbert (Jackie Jackson) wears one in "Three Loan Wolves," by the way.
I assumed that Joe wore one to look funny because the hat was worn (primarily) by children.