ESFPs, by definition, cannot be "intelligent", if we go with the traditional definition of "intelligence"; creative problem solving and analytical reasoning. If they are, but don't "show it", then they're not an ESFP at all, just an N masquerading as an S for some reason or another. MBTI doesn't make any revelations and it doesn't come to conclusions independent from what you've inputted to the test yourself. I'm not "generalizing" any more than someone is "generalizing" that everyone who answered "Yes" to "I am of African heritage" has black skin.
Are all ESFPs stupid? No. Are most? Probably. An ESFP, meaning an actual ESFP, not an N pretending to be or mistyped as an S, in higher education is there only because of their ability to memorize and regurgitate facts; not because of creative or original thinking. Again, I'm not generalizing; the DEFINITION of an ESFP is someone who is not interested in pursuing the "truth" of the matter and is content to party and dance. Just like an INTP is someone who is uninterested in small talk and social interaction, etc etc etc. The problem is that intelligence is so valued in our society, people don't want to acknowledge the fact that personality has more to do with intelligence than any other factor.
By the same token, an INTP cannot be "stupid" in the traditional sense. They may be disconnected from reality, paranoid, delusional, etc, but the DEFINITION of an INTP is someone who is interested in theoretical possibilities, knowledge, and abstract thinking (oversimplifying, obviously, but you get the point). Karl Pilkington is an INTP with a below average IQ, but he is still more intelligent than an ESFP with an IQ of 120 because of the way he arrives at his conclusions vs. the ESFP; the INTP forms abstract connections between concepts and uses logic to arrive at some sort of a result. The ESFP does what he does because of, again, rote memorization and regurgitation.