Imagine, you are on a random day trip orchestrated on a whim by your council president. Now, visualize wearing costumes as part of your speech mode-related gags.
Lastly, take a kigurumi somewhere along that equation.
Chizuru Akaba, perhaps the most intellectual of the group in terms of common sense thinking alone, decides that the best costume that would suit her onomatopoeic theme of choice... would be a sheep kigurumi.
To give context to this rather unusual situation, thanks to a bit of word slip by Minatsu, their always overeager student council president, Kurimu, orders everyone to embark on a short summer trip. This takes them to a train trip out of town, and onto their destination hot spring inn.
"Just shut up and follow what I say!"
After a round of the obligatory ghost story sharing session, Kirumu finally gives up and instead shifts their activities. Her brilliant suggestion, is to play out stereotypes with Japanese cliché speech habits. As to how exactly they got the costumes for that part, it is a mystery entirely on its own.
This is where the sheep kigurumi comes in. Chizuru apparently chose a speech habit of the informal onomatopeia mokyu, which immediately comes into mind something cute or cuddly. As to why a sheep was chosen..well, we guess "fluffy" was also one of the condition adjectives.
"Alright, how much do you want?" "Mokyu!"
In any case, this did a complete 180-degree turn for her as the usual cool, sadistic beauty plays a "fluffy" character. It was quite convincing though, winning the hearts of the entire student council, as well as winning a few Fukuzawas away from Ken.
"She looked cute when curled up, but now looks like a yeti when standing like that."
In the end, however, the kigurumi was eventually met with indirect disapproval, by pointing out that it looks kind of like a yeti suit when she does a T-pose.
Seitokai no Ichizon (生徒会の一存) may not be the pioneer, but it is one of the older series (as shown by the rather dated screenies) that takes the randomness factor and stirs it into something that the audience can enjoy. The anime adaptation even does a few direct jabs to roast its own source material just for your own entertainment.
In fact, how could even a series that mainly takes place in just one single room, on some random high school in Hokkaido, have two entire seasons out of it?
The kigurumi was just a single element out of the numerous once that the show manages to juggle together in a weird way, without becoming all too out of place.