Ugly looking != stupid looking.
I think we have identified a few elements of the stupid look but all of our evidence is anecdotal so any conclusions are going to be pretty shaky. A recent cow-orker of mine, a doughy faced mouth breather, gave me the impression he wasn’t very bright. He was in fact quite intelligent but his appearance fooled me. He turned out to be a arrogant, kvetching (sp?) prick which I would not have pegged him on just by appearance.
I recall seeing a quote attributed to Abe Lincoln, roughly, “By the time he’s 40, every man is responsible for the face he has.”
That’s kind of different than what the OP is asking, though.
I think there’s a dumb look to some athletes, characterized a little by a hooded brow, and kind of a “dumb mouth”. These guys sometimes just have that “slack” face. I guess you’d call it the basis of the dumb jock stereotype.
Although, it’s hard to be specific. . .
Steve Largent is a smart guy. He looks a little like Montana, but just a bit. . .brighter!
A nicer catch all term is “people with learning disabilities”.
But a catch all catches too much,
Albert Einstein, Winston Churchill, Alexander Graham Bell, Leonardo DeVinci all probably had the learning disability of dyslexia. Cite.
Personally I didn’t think Joe Montana or Clyde Drexler looked stupid from the pictures linked.
I have read a little about Fetal Alcohol Syndrome, and it seems to apply to this question. Children whose mothers drank a lot during pregnancy tend to score badly on intelligence tests. FAS victims also tend to have telltale facial features. One of these is wide-set eyes. I don’t remember the rest of those facial features.
So, in many cases, an adult with an “FAS face” will often have a lot in common with a box of rocks.
It’s hard to capture statically, I think.
Joe’s face is always emotionless, and it looks like he doesn’t understand what’s going on around him. And, I think his jaw hangs open a little. During an interview, sometimes it looks like it’s the first time he’s seen a microphone.
He almost looks like a guy who has had a lobotomy or been struck by lightning – just dull and dim.
Hmm… not having grown up with him (Rainman type situation coming up), I always tend to forget that I have a half brother that has FAS. I’ve only ever met him once, and lives in a special state-run home under custody of the state (NV). Highly reduced intelligence, facial and other physical deformations. I hate to sound like this, but I’m kind of glad I never got to know him because I’d feel, well, human towards him. 
“Sometimes” is probably the best answer.
The human brain is extremely good at pattern recognition, and is also very good at picking up indications that something is not quite normal. I find that I can detect when someone is not walking quite correctly due to an injury or neurological defect, just from watching them take a few steps. So I'm willing to believe that the brain can pick up on very subtle facial cues indicating either congenital problems or neurological damage. This probably works better if you actually watch the person for a few minutes and see how they respond to stimuli rather than studying a picture.
It’s easy to be fooled, though. Damage to the motor nerves (slurred voice, tics, slack muscle tone) can lead to a perception of lack of intelligence, even though they are unrelated to the intellectual capacity of the person involved.
We also tend to associate lack of maintenance (bad dental hygiene, ungroomed appearance) with lack of intelligence or at least lack of education, although those aren’t always the same thing.
And note that people often think dogs look intelligent because of their habit of cocking their heads and listening while gazing at you with bright, interested eyes, despite the fact that those dogs are not noticeably brighter than your average slack-jawed yokel.
We really have no basis for that conclusion, since we have no means of gauging canine intelligence. A genius who refuses to act like a genius is still a genius, and a retard who has adapted to live a normal life through imitation and disguise is still of reduced mental capacity. Dogs can be either.
According to this there is.
http://www.vcu.edu/uns/Releases/2005/june/061705.html
and on a related note
http://www.villagehatshop.com/facts_brain.html
-rainy
To me, she looks like she’s been beaten up. Her face is swollen and there are marks on her face that look like cuts and bruises.