It's cute and I liked it, but it's not something a girl in high school would wear. Or middle school. It's something little girls would wear. That's probably why it was cute on Koto: she's still a little girl in a lot of ways, especially emotionally.
I also liked the outfit Mariya picked out for her, although that also wasn't something a high school girl would wear. (Middle school might, so it was an improvement.)