Yes, LIGHTER is a Scrabble word!

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

Is it a valid word?

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

of comparatively little physical weight or density
(used of color) having a relatively small amount of coloring agent
of the military or industry; using (or being) relatively small or light arms or equipment
not great in degree or quantity or number
psychologically light; especially free from sadness or troubles
characterized by or emitting light
(used of vowels or syllables) pronounced with little or no stress
easily assimilated in the alimentary canal; not rich or heavily seasoned
(used of soil) loose and large-grained in consistency
(of sound or color) free from anything that dulls or dims
moving easily and quickly; nimble
demanding little effort; not burdensome
of little intensity or power or force
(physics, chemistry) not having atomic weight greater than average
weak and likely to lose consciousness
very thin and insubstantial
marked by temperance in indulgence
less than the correct or legal or full amount often deliberately so
having little importance
intended primarily as entertainment; not serious or profound
silly or trivial
designed for ease of movement or to carry little weight
having relatively few calories
(of sleep) easily disturbed
casual and unrestrained in sexual behavior

Also: light-colored, light-headed, low-cal, calorie-free.

Not playable in Scrabble US: LIGHTHEADED.