Yes, CARDINAL is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, R 1, D 2, I 1, N 1, A 1, L 1.

Is it a valid word?

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

(Roman Catholic Church) one of a group of more than 100 prominent bishops in the Sacred College who advise the Pope and elect new Popes
the number of elements in a mathematical set; denotes a quantity but not the order
a variable color averaging a vivid red
crested thick-billed North American finch having bright red plumage in the male

Also: cardinal number, cardinal grosbeak, richmondena cardinalis, cardinalis cardinalis.