Yes, MULLEIN is a Scrabble word!

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

Is it a valid word?

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

any of various plants of the genus Verbascum having large usually woolly leaves and terminal spikes of yellow or white or purplish flowers

Also: flannel leaf, velvet plant.