Yes, MINIMALISM is a Scrabble word!

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

Is it a valid word?

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

an art movement in sculpture and painting that began in the 1950s and emphasized extreme simplification of form and color

Also: minimal art.

Not playable in Scrabble US: REDUCTIVISM.