Yes, SPONGIEST is a Scrabble word!

Letter values in Scrabble US: S 1, P 3, O 1, N 1, G 2, I 1, E 1, S 1, T 1.

Is it a valid word?

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

easily squashed; resembling a sponge in having soft porous texture and compressibility
like a sponge in being able to absorb liquids and yield it back when compressed

Not playable in Scrabble US: SPONGELIKE.