Yes, HAMMERHEADS is a Scrabble word!

Letter values in Scrabble US: H 4, A 1, M 3, M 3, E 1, R 1, H 4, E 1, A 1, D 2, S 1.

Is it a valid word?

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

a stupid person; these words are used to express a low opinion of someone's intelligence
the striking part of a hammer
medium-sized live-bearing shark with eyes at either end of a flattened hammer-shaped head; worldwide in warm waters; can be dangerous

Also: hammerhead shark.

Not playable in Scrabble US: MUTTONHEAD, DUMBASS, FUCKHEAD.