Most of the information I've read about this says that tips do not correlate with job performance. Instead, things like race, sex, attractiveness, and other things have a bigger impact. I've even caught myself doing this, where I'll realize after the fact that the only reason I tipped extra was because I noticed the worker was either a very attractive woman/man. It's not even sexual attraction since I'm not attracted to men. My guess is that I'm more likely to notice good service if I first notice the server, which happens more when they are attractive.
It's likely similar to that bias where attractive people also progress faster in jobs. Even if you're not personally attracted to them, you still have that bias.
