Yes, MUDGUARD is a Scrabble word!

Letter values in Scrabble US: M 3, U 1, D 2, G 2, U 1, A 1, R 1, D 2.

Is it a valid word?

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

a curved piece above the wheel of a bicycle or motorcycle to protect the rider from water or mud thrown up by the wheels

Also: splash guard, splash-guard.