Yes, DECORATOR is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, C 3, O 1, R 1, A 1, T 1, O 1, R 1.

Is it a valid word?

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

a person who specializes in designing architectural interiors and their furnishings
someone who decorates

Also: interior designer, interior decorator, house decorator, room decorator.

Not playable in Scrabble US: ORNAMENTALIST.