Explain : "But for the stuffing, well, we all are mortal!"

Messenger: A lord to a lord, a man to a man; stuffed with all honourable virtues.
Beatrice: It is so indeed, he is no less than a stuffed man, but for the stuffing, — Well, we are all mortal.

In this exchange, Beatrice is mocking the Messenger's description. The messengers compliments because humorous barbs. The man isn't stuffed with virtues.... he's merely a stuffed man (a dummy). Beatrice ends by saying we're all mortal.... meaning imperfect.