Yes, FLATTER is a Scrabble word!

Letter values in Scrabble US: F 4, L 1, A 1, T 1, T 1, E 1, R 1.

Is it a valid word?

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

having a surface without slope, tilt in which no part is higher or lower than another
having a relatively broad surface in relation to depth or thickness
not modified or restricted by reservations
stretched out and lying at full length along the ground
lacking contrast or shading between tones
(of a musical note) lowered in pitch by one chromatic semitone
flattened laterally along the whole length (e.g., certain leafstalks or flatfishes)
lacking taste or flavor or tang
lacking stimulating characteristics; uninteresting
having lost effervescence
sounded or spoken in a tone unvarying in pitch
horizontally level
lacking the expected range or depth; not designed to give an illusion or depth
not reflecting light; not glossy
commercially inactive

Also: two-dimensional, 2-dimensional.

Not playable in Scrabble US: FLAVOURLESS, SAVOURLESS.