Yes, SOFTBALL is a Scrabble word!

Letter values in Scrabble US: S 1, O 1, F 4, T 1, B 3, A 1, L 1, L 1.

Is it a valid word?

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

ball used in playing softball
a game closely resembling baseball that is played on a smaller diamond and with a ball that is larger and softer

Also: playground ball, softball game.