Apparently, it's a stereotype, and it makes no sense to me. I understand how different personality types have different strengths and weaknesses, but how can you blame a person's level of intelligence on their personality? I think anyone could be a genius; it just depends on how hard they work and their willingness to learn. My mother, brother, and best friend are all ESFJs, and they are all very bright, intelligent people. I also know of an ESFJ who graduated valedictorian in a class of over 400.
Yes, when they get emotional or stressed, they don't really use logic or reasoning, but that's just because they are emotional. They aren't like that on a regular basis. This whole thing just confuses me.