Yes, SORRIEST is a Scrabble word!

Letter values in Scrabble US: S 1, O 1, R 1, R 1, I 1, E 1, S 1, T 1.

Is it a valid word?

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

feeling or expressing regret or sorrow or a sense of loss over something done or undone
bad; unfortunate
without merit
causing dejection

Also: good-for-nothing, good-for-naught, no-account, no-count, no-good.