Yes, MIZZENS is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, Z 10, Z 10, E 1, N 1, S 1.

Is it a valid word?

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

third mast from the bow in a vessel having three or more masts; the after and shorter mast of a yawl, ketch, or dandy
fore-and-aft sail set on the mizzenmast