At a colloquial level, and in a low register it does mean stupid. But it is the sort of word you might, in Britain, hear used in the crowd at a football match, or among the less sophisticated drinkers in a pub. It does not belong in an intellectual register. Educationalists have far better and more professional ways of describing people who are supposedly 'thick'.
It also depends on the dubious notion that there is a possible universal measure of intelligence, a particularly Anglo-centred idea, that is challenged by most modern schools of psychology.
Historically, intellectual capability was reduced to an index, known as IQ. Average was 100, with 10% of the population achieving scores of more than 125, based on spatial and diagrammatic tests.