From what I have seen of history, the most intelligent people (from what we have been able to tell) are never a regular part of the mainstream. They only dabble occasionally.
Are you saying Jefferson and Einstein are mainstream? That is clearly not the case. Their popularity is, but their actions were from from it. Hell, just LOOK at Einstein!
This recent post shot down a lot of things being mistakenly attributed to the Dunning-Kruger effect. Well worth the read, since it actually explains the study in a more comprehensive way, rather than the somewhat sensationalized version most articles have. Essentially, it says that the worst people judge their abilities to be on par with average people, thus grossly overestimating their abilities. The smartest people somewhat underestimate their abilities, but not nearly to the degree that the truly incompetent do.
I must admit, that article is way too long for me atm. Maybe that disqualifies me from posting here, but it also seems to me that the DK effect is more of a "fun fact" than a life changing realization. I have adopted the "I know that I don't know anything" wisdom long ago, and also subscribe to the "all code sucks" theory, so I try to shield myself from (what I think is) the DK effect a little bit.