Yes, MORTARS is a Scrabble word!

Letter values in Scrabble US: M 3, O 1, R 1, T 1, A 1, R 1, S 1.

Is it a valid word?

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

a muzzle-loading high-angle gun with a short barrel that fires shells at high elevations for a short range
used as a bond in masonry or for covering a wall
a bowl-shaped vessel in which substances can be ground and mixed with a pestle

Also: trench mortar.