Knygathin wrote:

I think Jack, dressed up with a napkin over his chest (besides keeping his clothes clean) has in mind to make a point: That ecstasy, and abandon to the pleasures of food (and Life!), is more important than adapting to inhibiting and silly etiquettes.

Also, he is being practical. For a gentleman with, how shall we say it, an expansive girth, a napkin on the lap is nuncupatory.