I am not sure the Female Changeling is stupid persay, rather her mind was being affected by the disease and the fact the Founder's have a psychopathic disregard for solids. Its part of the whole "Evil Cannot Comprehend Good" idea that often makes villains lose.
This attitude is seen in real life, during WWII the Nazis would destroy entire towns if the resistance movement in an occupied country was causing enough trouble. Heck the Nazis solution to almost any problem seemed to be as needlessly cruel as possible. The Nazis campaign in the East would have more successful, if the Nazis hadn't insisted on massacring and enslaving almost all the people in that region, making Stalin the better choice for the Russian population. You are pretty evil, if you can make Stalin look good by comparison. The Female Changeling just has that kind of warped outlook on life.