Actually, I'd think the quality difference of a Denny's vs. a five-star would inherently affect the quality of the server and the increase in tip. Foul-mouth Flo in her hairnet probably won't be worth more than 15% whereas Jacques with his silk tie is more likely to provide a 20% service.
Except that the bill at Jacques' restaurant is $100, so he gets $15. The Denny's bill is $20 and Flo gets $3. The 15% factors it all in already. The price of the meal at the nice restaurant is already including higher payroll cost for better servers. Jacques' silk tie gets him 5x more money and I don't have to change my percentage.