Yes, MESOMORPHIC is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, S 1, O 1, M 3, O 1, R 1, P 3, H 4, I 1, C 3.

Is it a valid word?

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

having a robust muscular body-build characterized by predominance of structures (bone and muscle and connective tissue) developed from the embryonic mesodermal layer