Yes, NAPKIN is a Scrabble word!

Letter values in Scrabble US: N 1, A 1, P 3, K 5, I 1, N 1.

Is it a valid word?

NAPKIN is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a small piece of table linen that is used to wipe the mouth and to cover the lap in order to protect clothing
garment consisting of a folded cloth drawn up between the legs and fastened at the waist; worn by infants to catch excrement

Also: table napkin.