East asian beauty standards. Pale skin is considered beautiful and can lead to toxic ways of thinking sometimes
Many kinds of standards can lead to toxic ways of thinking, in particular when it comes to superficial matters. Sometimes in unhealthy ways, if it's something that's outside healthy boundaries.
That can be very personal, since for instance some people just can't get as thin as others without starving themselves, while others (maybe the same "others" as before) could suffer from obesity related ailments before too many people would call them "fat".
but then a lot of manga tells about students who show off having tanned skin after a summer vacation at the beach
I think that's got more to do with the gap moe thing. It's a difference from the norm, so it's attractive when it shows up, but it's not generally seen as attractive as a permanent skin tone. The Western world used to have the same pale skin beauty standard, but the industrial revolution caused a lot of people to work indoors so much that it wasn't as rare for people to be pale, and following that the beauty standards started tilting the other way instead. The Asian world didn't (to my knowledge) have that shift, so they kept their old standards.