Turns out quite a bit
What does make us tip better? How about breasts? A 2009 study published in the Archives of Sexual Behavior found that larger self-reported breast size among waitresses correlated with bigger tips. Similarly, a 2010 study in the International Journal of Hospitality Management found that waitresses' use of makeup significantly increased their tips. Female servers can also increase their tips by drawing a smiley face on the back of customers' checks (male servers, it turns out, can't achieve the same with this tactic).
Only a part of the article, it it gets to the main point. A large influence on how much/if we tip is based on appearance, especially in men. This helps enforce the traditional sex/gender roles by making it so that women have to look 'pretty' in order to get good tips, combined with some states allowing companies to have lower than minimum wage in tip based services mean that women in these jobs have to cater to what other people think look good in order to get paid. I can see correlations to the sports field where often times the publicity given to an athlete has much more to do with her looks than ability.