Yes, MIMOSA is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, M 3, O 1, S 1, A 1.

Is it a valid word?

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

evergreen Australasian tree having white or silvery bark and young leaves and yellow flowers
any of various tropical shrubs or trees of the genus Mimosa having usually yellow flowers and compound leaves
a mixed drink containing champagne and orange juice

Also: silver wattle, acacia dealbata, buck's fizz.