A possible positive aspect is that it can serve as a tool for character development. The mature woman wearing a school girl outfit could be on a journey of self - rediscovery. It might show her trying to reconnect with a part of herself that she lost over the years. For instance, she could be a businesswoman who has become too serious and rigid, and the school girl outfit helps her loosen up and find her inner creativity again in the story.
One positive aspect is that it can be used to break stereotypes. Usually, we associate school girl outfits with young girls, but when a mature woman wears it in a story, it challenges our preconceived notions. It can also add an element of fun and playfulness to the story. Maybe it's part of a costume party in the story, and the mature woman's choice of the school girl outfit brings laughter and joy to the event.