So this trope/cliche is possibly one of the most common out of all the tropes and cliches. Now Planet of the Hats basically means there is one defining trait/characteristic among a planet/group of people. This normally is found in the sci-fi genre but can transfer over into the realm of fantasy. Overly Sarcastic Productions can explain this better or in more detail than I will go into.
Now when I saw trait or characteristic, it can be a number of things. The people are matriartical society, they are all excellent swimmer/sailors, they all have blue hair, they don't understand figurative language and take everything literally, so on and so forth. And it does sometimes just apply to sci-fi, because most sci-fi media takes place in space and they're exploring some galaxy somewhere. It's often an excellent tool when it comes to tackling social issues going on in the world, though it's not allows noticeable at first.
I'm sort of on the line with this due to many reasons. One being that the POV character and/or hero of the story often thinks of themselves as an outsider among their own people. If everyone they grew up with knows different levels of magic and they can't even cast the basic spell, they in turn develop some other skill such as swordsmanship or whatever. If anything it's just used for world building. And before you ask, world building is basically creating the location of the story/game/show/etc. with specific regions, races, creatures, etc. One very well known example of this is Lord of the Rings, The Hobbit and everything else JRR Tolkien. Most of the inspiration comes from Norse mythology, but everything is pretty defining.
There are way too many subtropes for this to cover in just one chapter, so I'll cover a few now, and return with more chapters on them.
-The Dragonball series gives us two Planet of the Hats, Saiyans and Namekians. The Namekians are green, reproduce asexually, is one gender (male), have mystical abilities, and is peace-loving with the exception of Piccolo who was the Nameless Namek's evil side that was corrupted by living on Earth.Now there is little difference among them except for the clans. There are those of the mystic Dragon clan (who are the guardians of the dragonballs) and the Warrior clan( whose name spells it out for you). And then there are the Saiyans, who are a race of barbaric warriors who spend most of their time killing other races for funsies. All Saiyans are freaking strong, transform into giant apes under a full moon, are prideful, and are Blood Knights(people who just want a good fight).
-Going back to the sci-fi thing both Star Wars and Star Trek are living embodiments of this trope. Both have planets/races that have specific traits or characteristics.The Vulcans are all logic all the time. So on and so forth...I like this phrase.
-Pokémon seems to be a Planet of Hats as well — much of culture and society revolves around Pokémon, from the economy (shops and huge department stores which sell only Pokémon-related goods) to the government.
Now when writing fantasy or whatever using this trope, be careful. The last thing you need is for monoculture. There is nothing wrong with it, it's just boring. There are ways to mix it up a bit. Let's say the region our hero(es) are in is close to a beach/open water, instead of making everyone OP sailors , why not use it to build the culture around it. Like certain festivals or superstitions ( like wearing a hat on a boat will cause seasickness). What is the government like, do they farm, what is the military like ( if they have one), etc. Don't just focus on that one defining trait or characteristic because if there is an important character in the town that they are visiting, some people would like some backgrounds/ clarity on why the hero is about to pushed into an active volcano for accidentally breaking a sacred item.