Why is it important that Shelley mentions Da Vinci in his poem's full title, On the Medusa of Leonardo Da Vinci in the Florentine Gallery?

Because Leonardo Da Vinci was of many Da Vinci that created art. Another reason would be that in early Italy during the renaissance. It was only proper to use full on going names to greet, cast farewells and to mention in papers or documents. Also if Your giving credit to someone for their work, if they sign their full name on their work, you should too out of respect.