Yes, NIM is a Scrabble word!

Letter values in Scrabble US: N 1, I 1, M 3.

Is it a valid word?

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

game in which matchsticks are arranged in rows and players alternately remove one or more of them; in some versions the object is to take the last remaining matchstick on the table and in other versions the object is to avoid taking the last remaining matchstick on the table