This doesn't make any sense. Firstly, it's common practice (the norm) for tips to be pooled and split among all staff, so it doesn't, in fact, result in good employees being paid more than worse ones.

Secondly, solving the principal-agent problem is the employer's problem, and they could do that by collecting feedback, paying bonuses, firing people, etc. Punting the whole thing to clients just means that it becomes a lottery based on the client's mood, wealth, expectations, instead of something objective.