You can say whatever you want about the rationality or irrationality of Trump himself, but cognitive science does not support the claim that his voters were irrational—or, more specifically, that they were any less rational than the Clinton voters. Politics is not the place to look for objective rightness or wrongness—and that is what judgments about the rationality of voting entail. Our judgments in this domain are uniquely susceptible to myside bias.
Many of our most contentious political issues hinge on values and culture rather than facts. That may be a good thing. It could be signalling that our society has already handled the easiest issues—those that can be solved by educating everyone to accept the same facts and then implementing the obvious solution that follows from these facts.