Yes, SIDEBURNS is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, D 2, E 1, B 3, U 1, R 1, N 1, S 1.

Is it a valid word?

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

facial hair that has grown down the side of a man's face in front of the ears (especially when the rest of the beard is shaved off)

Also: mutton chop, side-whiskers.

Not playable in Scrabble US: SIDEBURN, BURNSIDE.