I prefer to think that most people who espouse creationist ideas are just lacking in a solid understanding of science, and continue to receive misinformation—as opposed to lacking in actual intelligence. In other words, I prefer the word “ignorant,” which while pejorative, still has its valid uses, to “stupid,” which is little more than an insult.
It’s getting harder and harder to tell which is which these days, though.
The world is filled with information, available to all who seek it. At some point, individuals have to assume some level of responsibility for the information they consume and the ideas they espouse. While “stupid” is little more than an insult, it’s often the correct characterization. Give people enough credit to inform themselves or wallow in stupidity. It’s a choice.
One can be intelligent and thoroughly exposed to the data and reasoning behind the science, and STILL believe in creationism.
Confirmation bias is a powerful thing. If creationist beliefs are deeply ingrained and culturally reinforced by one’s community, the mind sees them not as beliefs of faith but as known facts. One is practically an eyewitness to that “truth.” The scientific data and reasoning aren’t evaluated according to how well they explain/predict stuff, but how well they support what is already known to be true.
It’s not only creationists who do this. In fact, I’d go so far as to say creationism is one of the more harmless varieties of confirmation bias. Ditto for flat-earth types and such. Few lives are ruined because someone was wrong about how old the Earth is or how life has evolved. Much more dangerous are anti-vaccination types, homeopaths, and FBI agents looking for evidence that THIS guy did it.