Yes, MOCKER is a Scrabble word!

Letter values in Scrabble US: M 3, O 1, C 3, K 5, E 1, R 1.

Is it a valid word?

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

someone who jeers or mocks or treats something with contempt or calls out in derision
long-tailed grey-and-white songbird of the southern United States able to mimic songs of other birds

Also: mimus polyglotktos.