We must never forget that half of voters are even dumber than the average voter
Also, human IQ is distributed in such a way that the mean is approximately also the median, which, by definition of IQ, is 100.
The reality is if you measured absolute intelligence that it would not be a standard distribution (there are more people at the bottom than expected), but it would be close. The reason why is that intelligence is the result of tens of thousands of independent factors (mostly genes).