"Fat-headed" is sometimes used to describe someone who is egotistical-- that is they have too high of an opinion of themselves. The other two are really old-fashioned ways of calling someone stupid. All three usually refer to individuals-- not groups or actions. None of them really fit the tone of your example sentence very well. I would suggest: The company's decision to enter overseas markets without any research was quite imprudent. (very formal) The company's decision to enter overseas markets without any research wasn't very well thought-out. (a little less formal)