Yes, RIGHTER is a Scrabble word!

Letter values in Scrabble US: R 1, I 1, G 2, H 4, T 1, E 1, R 1.

Is it a valid word?

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

being or located on or directed toward the side of the body to the east when facing north
free from error; especially conforming to fact or truth
socially right or correct
in conformance with justice or law or morality
correct in opinion or judgment
appropriate for a condition or purpose or occasion or a person's character, needs
of or belonging to the political or intellectual right
in or into a satisfactory condition
intended for the right hand
in accord with accepted standards of usage or procedure
having the axis perpendicular to the base
(of the side of cloth or clothing) facing or intended to face outward
most suitable or right for a particular purpose
precisely accurate

Also: right-hand.