From the Super Mario Wiki
Mona's Mirror is an item in the SNES release of Mario's Time Machine. It is a mirror owned by Lisa del Giocondo, the subject of Leonardo da Vinci's famous painting, Mona Lisa. After a visit, she leaves it in Michelangelo Buonarroti's workshop. Mario later asks about her after giving Michelangelo a Bucket of Plaster, which leads him to remember the mirror. He then gives it to Mario and asks him to return it to Lisa. However, one of Verrocchio's apprentices had previously asked for a mirror in order to read the mirror writing in Leonardo da Vinci's Notebook, and as Mario is unable to return the mirror to Lisa, he instead gives it to the apprentice. The apprentice uses the mirror to read the Notebook's contents and describes its contents to Mario. The apprentice keeps the mirror, preventing Mario from returning it.