Yes, DIMINUTION is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, M 3, I 1, N 1, U 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

change toward something smaller or lower
the statement of a theme in notes of lesser duration (usually half the length of the original)
the act of decreasing or reducing something

Also: step-down.