Yes, TIPPER is a Scrabble word!

Letter values in Scrabble US: T 1, I 1, P 3, P 3, E 1, R 1.

Is it a valid word?

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

a person who leaves a tip
truck whose contents can be emptied without handling; the front end of the platform can be pneumatically raised so that the load is discharged by gravity

Also: dump truck, tipper truck, tipper lorry, tip truck.