Planet of Dunce Caps

Here's a Planet of Hats in which the hats are on empty heads. Planet of Dunce Caps is when an entire species seem to be less than intelligent. If Humans are the idiots, then it's Humans Are Morons.

Sister Trope to Insufficiently Advanced Alien.

Comic Books

 * Sillage: a species developed as clone soldiers with minimal intelligence. Even when freed, the one intelligent member of their race says they don't know what to do with themselves.
 * The goblins in House of Mystery aren't a bright lot, with one character being an exception.
 * A strange case in Marooned as there is very brilliant species thats brains in robot bodies. They couldn't manage to land their highly advanced ships. When one ship was saved, two of those died by accident while exiting the ship. Out of all the crew in ship, only one survived.

Film

 * While not stupid, the little three-eyed Martians from Toy Story seem to be quite gullible.
 * Humans might be considered this if you've seen the movie version of The Hitchhiker's Guide to the Galaxy. That might be because Humans Are Morons.
 * In Mom and Dad Save the World, the inhabitants of the planet Spengo aren't too bright.
 * Earth's future in Idiocracy. Apparently everyone becomes a moron.
 * The aliens in Galaxy Quest are pretty childlike and innocent, which more than offsets their native intelligence and technological level.

Literature

 * Posleen War Series: The Posleen are an incredibly stupid variant of the Horde of Alien Locusts. This was done in order to make space aliens that can attack but still be defeated.
 * The ogres in Xanth are stupid and proud of it.
 * Discworld has the Stupid Lizard Men. That's actually what they're called. They're all called Slime, since they can't remember any other names. As of The Last Hero there was believed to be only two left, but over the course of the book there was a battle in which they were both killed. By their own weapons.
 * In Nine Princes In Amber, Corwin forms an army by recruiting entire species of creatures that are brutish, but a little slow. They also happen to have a predilection toward hero worship. This is all justified by the fact that as a Prince of Amber, Corwin can walk through Shadows until he finds one with an ideal species to recruit.
 * The Hork Bajir from Animorphs were designed by the species that created them (to be gardeners of a sort) to be peaceful and not terribly bright. Occasionally one is born with human-level intelligence though.

Live-Action TV

 * The Star Trek: The Next Generation episode "Samaritan Snare" featured the Pakleds, who appear to be functionally mentally disabled, though they might be more of a case of Obfuscating Stupidity.
 * The Ogrons of Doctor Who are explicitly Too Dumb to Fool.

Tabletop Games

 * Dungeons & Dragons has many mookish monsters with their mental stats lowered in this way.
 * Orcs have lower Intelligence and Wisdom caps in AD&D2 and penalties to intelligence in 3.5 Dungeons & Dragons. Lizard Folk, Minotaurs and most goblinoids have the same or worse hindrances. Ogres are dumb, but have also Oni-like Ogre Magi race who tend more toward "devious" and look down on those brutes.
 * Ogryns in Warhammer 40000: those who show an exceptional level of intelligence (something like a four-year-old's) are surgically enhanced to be marginally smarter. They all still come across as Psychopathic Manchildren, though.
 * Orks in the same setting - being the comedy relief, they go for Too Dumb to Live with great regularity, though they are also brutally cunning on the rare occasion that they aren't cunningly brutal.

Video Games

 * Doogs from Star Control 3.

Western Animation

 * Considering each sample of DNA in Ben 10's Omnitrix represents a species, Rath's race seems to be superstrong, arrogant, and kind of stupid.
 * The Jackoffasaurs from South Park (which were an obvious parody of Jar Jar) are a race of morons.
 * The Ogres in Adventures of the Gummi Bears are all pretty dumb, bordering on Minion with an F In Evil. The one smart Ogre, Toadwort, is about as tall as a human five year old and is routinely abused by his brother Ogres and his boss, The Man Behind the Monsters, Duke Igthorn.
 * In one episode of the Mr. Bean animated series, it's revealed that Mr. Bean is actually from a planet where everyone looks and acts exactly like him.