Yes, SIMPLISM is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, M 3, P 3, L 1, I 1, S 1, M 3.

Is it a valid word?

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

a simplification that goes too far (to the point of misrepresentation)
an act of excessive simplification; the act of making something seem simpler than it really is

Also: oversimplification.