He does... neither, really, choosing a third option and ignoring the plate's existence for now.

"No, because Cora-san polite," Law responds flatly — it's true, too. Cora-san actually cares about people, and what they think. Law... not so much.

With his eyebrow raised, "It could mean mister... but it could also mean hey you."

Do with that information what you will... or probably more with the fact that he actually calls Rosinante Cora-san, the only actually polite way of address he has in his vocabulary.