Yes, SUCKER is a Scrabble word!

Letter values in Scrabble US: S 1, U 1, C 3, K 5, E 1, R 1.

Is it a valid word?

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

a person who is gullible and easy to take advantage of
a shoot arising from a plant's roots
a drinker who sucks (as at a nipple or through a straw)
flesh of any of numerous North American food fishes with toothless jaws
hard candy on a stick
an organ specialized for sucking nourishment or for adhering to objects by suction
mostly North American freshwater fishes with a thick-lipped mouth for feeding by suction; related to carps

Also: fall guy, soft touch, all-day sucker.