-They are stupid.
-They are xenophobic racists (over 50% of the population!).
-Trump welcomes the Brexit, so being pro-leave is stupid.
-Old people vote "leave" since they don't have to face the consequences as long as the young people, and old people have no clue.
-"Leave" votes come mainly from E.U. subsidized areas, thus they bite the hand that feeds them.
-Pro-leave doesn't even know what the E.U. is.
I have read a lot of insults regarding either position, but the attribution of stupidity comes almost always from the "Remain" camp. Which comes close to saying: there are no reasons to vote for leave, so they must be stupid. How stupid.