Yes, PLURALITY is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, U 1, R 1, A 1, L 1, I 1, T 1, Y 4.

Is it a valid word?

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

the state of being plural
a large indefinite number
(in an election with more than 2 options) the number of votes for the candidate or party receiving the greatest number (but less that half of the votes)

Also: large number, relative majority.