Yes, PUMPKIN is a Scrabble word!

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

Is it a valid word?

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

a coarse vine widely cultivated for its large pulpy round orange fruit with firm orange skin and numerous seeds; subspecies of Cucurbita pepo include the summer squashes and a few autumn squashes
usually large pulpy deep-yellow round fruit of the squash family maturing in late summer or early autumn

Also: pumpkin vine, autumn pumpkin, cucurbita pepo.