Scout tells us that, according to aunt alexandra, 'everybody in maycomb, it seemed, had a streak: a drinking streak, a gambling streak, a mean streak, a funny streak.' what is a 'streak'?

A streak is something that characterizes somebody - an element of somebody's character that practically defines them.

So, a drinking streak refers to a person who has a drinking problem; a gambling streak refers to someone who gambles a lot; a mean streak is a bad and a mean person; and a funny streak is a humorous person.