Look below!

Everybody has been a Goober at some point in their life. We have all done things that look like we possess no intelligence at all or very little, when we all do possess some.
Goober is not a slur against people lacking in intelligence. But a way to laugh at ourselves for the silly things we sometimes do (even though some goober jokes feature things so stupid nobody would do them). Goober is a term that gets substituted into jokes that feature blondes, terms of ethnicity, and the like. That way we can laugh without having to risk hurting someone.
So when you see Goober in a joke have a laugh - maybe even at yourself.