I doubt "pure" is the best word to describe 清楚, but it does mean something along the lines of "simple", "innocent", "clean", "plain", "tidy", and so on. Please refer to that YouTube video or Google Image Search for the connotation of the word. As a matter of fact, 清楚 is a common word that virtually every teenager understands, and it has been around at least for 100 years without any significant change in the meaning. I'm curious what page said this term is VTuber-derived. Recently, this word may be especially common as a category of the faces and personalities of young female people (including idols and VTubers), but it is not used in a different way from its original meaning.
EDIT: As pointed out in the comment section, the word 清楚 seems to be often used sarcastically in the VTuber community, and the word seems to have been imported to fans in the West along with the incorrect definition. See this article. This is a kind of local joke that only works in the VTuber community.