Eh, some of us feel self-conscious showing lots of leg, or don't like tight and short skirts. I'll happily wear a nice dress to a club or bar but at work I dress pretty formally, with a lot of knee-length skirts. It's what I feel comfortable in, even if it's not 'mature' or fashionable. I do kinda resent that people (not you, but some others I've seen online) think women should either look super sexy or like Christian grannies, no in-between. Ruby's clothes are just baggy and ill-fitting and emphasize how small she is, so they give her that 'childish' look - and the school badge and Victorian child details don't help. It's not about the length, it's more the general cut of her clothes, like they're borrowed from someone older or younger.