I wouldn't say "stupidity" is a hallmark of Thai culture, but willful ignorance- in a variety of circumstances, for example when facts are unpleasant or dangerous, for the sake of civility or politeness, etc.- might be. There is also a tendency to discourage/general reluctance to engage in critical thinking which I think is encouraged by Buddhism- and sometimes it really is better not to "think too much." (Also, too, the education system here sucks.) These characteristics are hardly unique to the Thais (the Japanese are not usually considered stupid, but the lack of critical thinking there can be surprising), but they are pretty good at not knowing/not thinking, and it can be part of the charm of the place, though maybe not so much when you are living here as opposed to holiday-making.