Why Ruyi cut your hair was so serious? I feel like I'm the only one with that doubt lol

Hair has very significant social and political value in ancient China.

1. From a Confucian point of view, cutting one’s hair was considered unfilial.

From Wikipedia:

It is for this reason that hair cutting is sometimes used as punihsment in ancient China. To have your hair forcibly cut is a great humiliation.

When the Manchurian first took over China and established the Qing dynasty, they had to enforce the Manchurian queue hairstyle with threats of death for it to be complied with. Many did die for protesting this. (Republican-era dramas/movies about politics will always have a symbolic™ scene of a man cutting his queue.)

2. In Manchurian culture, aside from the men who shave the front of their heads, you would only ever cut your hair in time of deep mourning. For a woman of Ruyi’s status (the empress) she could only cut her hair when either the emperor or empress dowager died.

3. Culturally, marriage is conceptualised by a couple’s hair being tied together. In wedding ceremonies there is usually a part where a lock of the husband’s hair and a lock of the wife’s hair is tied together to symbolise unity and the fact that they are bound. In everyday language, marriage/husband and wife are described as 结发夫妻 or “husband and wife whose hair is tied together”. In the drama, Qianlong always describes Langhua as his 发妻 (lit. hair wife) to signify that she is his official wife.

In the drama, Ruyi’s intent in cutting her hair was to signify the death of her marriage and the fact that she no longer wish to remain Qianlong’s wife. Basically it’s a statement of divorce. However, it can also be read as cursing death on the emperor and/or empress dowager, which is basically treason. (I mean wanting to divorce the emperor is pretty much treason as well…) That’s why it is considered an earth-shattering thing, and to this day, people still debate about why the historical Step Empress (was said to have) cut her hair since it was such taboo.