How smart do you reckon high-end wizards have to be, then ?
I tend to go by rule of thumb comparing the amount of time comparing wizards' experience with magic with the sort of investment of smarts it takes to get to a similar position working in science, that being the field I know well, and Thomas does compare Harry's skill level in "Backup" to someone with multiple PhDs iirc. And that's before you consider adding in literal centuries of experience,
Experience means learning from your mistakes, sometimes your own stupidity. We are watcheng Harry and Elaine doing exactly that. In matters outside wizardry Karen as a vanilla mortal is more experienced than Harry. And not less intelligent.
or entities that are described as, and depicted consistently with, superhuman in some extents.
(Besides, everyone knows wizards rely on INT as their primary ability score.)
How stupid can Harry be? Admittedly Elaine is more intelligent than Harry but her experience with the real world is in Summer Knight probably rather limitted and what she knows was for a large part controlled by Justin and a bunch of Sidhe. Ignorance is not the same as stupidity but it can look quite similar sometimes.
Harry and Elaine were not that old at Summer Knight so extra wizard experience is not a factor, we can assume normal human behaviour so what causes stupid behaviour in intelligent people?
Overspecialization can be a factor. People are not always that smart outside their field because it takes most of their brainpower to be sometimes extremely good in what they do.
Wishfull thinking is another one. Sometimes information is so unwelcome that you ignore it or go for the most convenient option for yourself on the short term. It is related to short term self interest which can lead to stupid rationalizations that are really believed by even intelligent people. I think Elaine fell victim for that one. Easily if Aurora was her main source of information.
Addiction can lead to stupid decisions and intelligent people can rationalize them, those rationalizations sound stupid to not addicted people.
Youthfull indoctrination can lead to stupidly holding irrational beliefs at a later age which can influence everything.
Extreme success can lead to overconfidence especially if you have power because nobody wants to disagree with you.
All above factors and more can make even intelligent people slip into intellecual laziness sometimes especially if they get older.
If they get older they rely more on experience and less on thinking things over as well which works quite well most of the time but can lead to stupidity if circumstances change.
And sometimes you are just tired, emotionally exited or overstressed. Everyone can make a stupid decision sometimes especially under pressure. Stupid people make them all the time.
Intelligence is often a protection against stupid or seemingly stupid decisions but not always.
https://en.m.wikipedia.org/wiki/Cold_fusionWe were studying chemistry at Leiden University at the time and it seemed wishful thinking to us but it went on for a long time even when it was utterly discredited in the rest of the scientific world. All people involved were intelligent people, most of the time. I was fascinated by it not as science but because how it could go on for such a long time publication after publication. Even a PhD is not always a protection against stupidity.