Yes, SCREWBALL is a Scrabble word!

Letter values in Scrabble US: S 1, C 3, R 1, E 1, W 4, B 3, A 1, L 1, L 1.

Is it a valid word?

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

a whimsically eccentric person
a pitch with reverse spin that curves toward the side of the plate from which it was thrown

Also: nut case, half-baked.