Yes, WIMP is a Scrabble word!

Letter values in Scrabble US: W 4, I 1, M 3, P 3.

Is it a valid word?

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

a person who lacks confidence, is irresolute and wishy-washy
a hypothetical subatomic particle of large mass that interacts weakly with ordinary matter through gravitation; postulated as a constituent of the dark matter of the universe

Also: weakly interacting massive particle.