Yes, TOP is a Scrabble word!

Letter values in Scrabble US: T 1, O 1, P 3.

Is it a valid word?

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

the upper part of anything
the highest or uppermost side of anything
the top or extreme point of something (usually a mountain or hill)
the first half of an inning; while the visiting team is at bat
the highest level or degree attainable; the highest stage of development
the greatest possible intensity
platform surrounding the head of a lower mast
a conical child's plaything tapering to a steel point on which it can be made to spin
covering for a hole (especially a hole in the top of a container)
a garment (especially for women) that extends from the shoulders to the waist or hips
a canvas tent to house the audience at a circus performance

be superior or better than some standard
pass by, over, or under without making contact
be at the top of or constitute the top or highest point
be ahead of others; be the first
provide with a top or finish the top (of a structure)
reach or ascend the top of
strike (the top part of a ball in golf, baseball, or pool) giving it a forward spin
cut the top off
be the culminating event
finish up or conclude

Also: top side, upper side, top of the inning, spinning top, circus tent, big top, round top, go past, top out, top off.