One hand washes the other and as a waiter, you need those people on your side.
Never heard of a customer doing this directly to the kitchen staff.
Our son has worked almost every job in food service, from prep, to expo, to waiter, and bartending also. At one of the places he worked, there was a menu item to send a round of beer to the staff to be enjoyed after hours. Since we were close to many of the staff, we did order a round for them occasionally. Our son said it was always appreciated and enjoyed at the end of a long day, not during it.