Turns out, Mary Sues come in all shapes and sizes.
This anime is a weird thing to judge. It has barely any conflict. Normally, this is fine for a slice of life story. The bigger issue, is that the conflict that is there is resolved very easily. The driving plot of her wanting to read a book is broken down in to lots of sub-parts (making paper, making wood blocks, etc.). But it means very little.
The characterization is also very awkward. The main character is your typical isekai protagonist, so her behavior is appropriate. But all the other children in the show act like they are triple their age. And this is not explained away by the medieval nature of the setting.
The story and characters and world are all fine. They are watchable and I laughed out loud at several parts. But know going in that this is a VERY light work.