In Chinese culture, heavily influenced by Confucian values, wisdom (智慧, zhìhuì) is one of the highest virtues. A respected person is expected to be thoughtful, prudent, and considerate. 愚蠢 (yúchǔn) is the direct opposite of this ideal. To be called 愚蠢 is not just an attack on your intelligence (IQ), but a condemnation of your character and judgment. It suggests you have failed to cultivate the basic wisdom expected of a mature individual. A helpful comparison is to the English word “foolish,” especially in its older, more biblical sense. When the Bible speaks of “the foolish man who built his house on the sand,” it's not saying he had a low IQ. It's a moral and character judgment about his lack of foresight and wisdom. 愚蠢 (yúchǔn) carries a similar weight. It's far more serious than a casual English insult like “stupid” or “dumb,” which can sometimes be used lightly or playfully among friends. Calling someone 愚蠢 in Chinese is a grave insult.
愚蠢 (yúchǔn) is a strong, negative term used to express serious disapproval or frustration. It is almost never used playfully.