There is never a stupid decision in selling bitcoin, it is purely down to individual beliefs and the way in which one wishes to deal with the market as it is. Of course, as with any currency, bitcoin as a crypto currency is liable to peaks and troughs, and examining the market is the best way to make a decision on selling.
Selling your bitcoin in the right time and in the right value is not stupid but if you buy said bitcoin in very expensive value but due to the market price bitcoin is decreasing dn you will get panic and sell it in lower price, maybe we call it stupidity, because we invest in this to earn profit not not to lose.