Yes, MIDRIFF is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, D 2, R 1, I 1, F 4, F 4.

Is it a valid word?

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

the middle area of the human torso (usually in front)
(anatomy) a muscular partition separating the abdominal and thoracic cavities; functions in respiration