Seriously now, the belief in fennel being good for the eyes must have derived from the name, finocchio, not the other way around. Sure, the "eyes" in the roots of the drawing might be a result of that folk etymology. And again they might not.
And what about the tiny little dragon frolicking between the roots of another plant on I forgot which folio?
"Finocchio" BTW, also means "pederast" in Italian.