I think calling it mere stupidity is a little too reductive. There are genuine grievances among his supporters, such as rising inequality, loss of opportunities/jobs and an economic system which is not working out for them. But expecting a narcissistic misogynistic racist billionaire rapist to actually help them is…the definition of stupidity.
At the end of the day though he was the one that spent the time to understand what they care about.
> Literacy levels: 54% of adults read below a 6th-grade level, and 20% read below a 5th-grade level.
This is the reality in America. The education system failed these people. Trump is merely taking advantage of that. He understands that logical arguments aren't necessary, merely emotional ones that appeal to how downtrodden and forgotten these folk feel. If he can make them believe that building a wall and/or deporting immigrants will get them their jobs back or that tariffs will bring manufacturing back to America that is more important to winning an election than truths or reality.
He won fair and square, the election -is- a popularity contest, not a competency contest.