Yes, MELLOWEST is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, L 1, L 1, O 1, W 4, E 1, S 1, T 1.

Is it a valid word?

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

unhurried and relaxed
having a full and pleasing flavor through proper aging
having attained to kindliness or gentleness through age and experience
having attained to kindliness or gentleness through age and experience
slightly and pleasantly intoxicated from alcohol or a drug (especially marijuana)

Also: laid-back.