Yes, CUMQUAT is a Scrabble word!

Letter values in Scrabble US: C 3, U 1, M 3, Q 10, U 1, A 1, T 1.

Is it a valid word?

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

any of several trees or shrubs of the genus Fortunella bearing small orange-colored edible fruits with thick sweet-flavored skin and sour pulp

Also: kumquat tree.