Yes, MITIGATION is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, T 1, I 1, G 2, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

to act in such a way as to cause an offense to seem less serious
a partial excuse to mitigate censure; an attempt to represent an offense as less serious than it appears by showing mitigating circumstances
the action of lessening in severity or intensity