yeah, it doesn't make as much sense for nyo!japan, haha. Kiku not having long hair in canon before the meiji-era feels like a missed opportunity precisely because for Japan (and China), the politicisation of short hair with modernisation, westernisation and "civilisation" in the 19th century (or: from the point of view of their opponents, a rather disgraceful break from important traditional values)—was centred on men's hairstyles, rather than women's. after all, it was still the norm for western women to have long hair in the 1850s. short women's hair as a statement emerges as a global trend more in the 1920s—and Japanese women adopted those styles too (as these photos show).
for men, short hair wasn't really regarded as appropriate in japan or china before the modern era. there were some exceptions, like shaving your head bald to enter a religious order. but otherwise from what i know, forcibly having your hair cut could be a punishment, precisely because the norm is long and pulled back into a bun/topknot, or the Manchu-style braid/queue in Qing China (which was its own controversy because the Qing rulers were ethnically Manchu, and the queue was seen as a foreign imposition on Han Chinese culture). Nonetheless, overall trend = long hair for men.
helltalia-wise, i headcanon that both Kiku and Yao had dramatic haircuts, though Kiku did it first (by 1867, when the Meiji emperor ascended the throne)— in line with how his long-term, pretty intimate relationship with Jan/Johan (Ned) and his exposure to Dutch studies pre-disposed him to being more willing to adopt western styles faster than Yao. after all, Kiku's no stranger to borrowing from others. i like the visual break from tradition and change in worldview this represents, especially since Kiku was not only heavily influenced by Yao for more than 1000 years before (the kimono itself was adopted from the traditional chinese hanfu), but because long hair for men also connoted "civilisation" through the chinese imperial worldview. for example, when china was conquering and colonising what is now southern china, the baiyue tribes who lived there were perceived as 'barbarians', and chinese writers back then noted their short hair as a point of distinction.
so, Kiku cutting his hair short is both about reinventing himself, but also inherently embedded with a symbolic turning away from Yao in some ways (as @hetagrammy and @acemapleeh also observed with some solid tag comments about Confucianism, the mentor-protege dynamic these two had vs. the pull of westernisation). it's like, Yao and Kiku have been at war with each other before the Westerners showed up (it involves Yong Soo, Yao kicking Kiku's ass and being very 'remember your place, upstart'), but to me, what still felt constant even during such periods of antagonism is that it's implicitly taken for granted that China was THE powerful empire, and that Yao's cultural traditions connoted power and prestige—in a manner similar to the sort of image Europeans had of Rome and Greece. so i really like the visual of Kiku's Dramatic Haircut, especially since one reason Japan eventually caved to the Americans' demands during the Perry Expedition was that they'd already heard from the Dutch what happened to China during the Opium Wars.
