Here at the Jersey Shore we get plenty of "Guidos/Guineas" during the summer. Does that mean
all Italians (of which I consider myself)? Of course not.
But if someone of seemingly Italian decent comes down from North Jersey or New York driving a Z28 wearing a "guinea tee" shirt, driving like a dick and tossing trash out the car window, and dripping Patchouli cologne from his hairy back listening to 80s rock; then yeah, he's a Guido.

lol (They exist.)
Stereotypes are based on truth, and many perpetuate the stereotype. But that doesn't mean every Italian is a "Guido" -- same for all races/creeds/etc.