Most Helpful Guy
It means that the girl is physically attractive, but she either lacks a trait that I want or she possess trait that I don't want.
For example, If a pretty girl who is known to be very religious walks into my classroom, then I'll classify her as someone who "is good looking but is not my style". It means that while I do find her to be very attractive, the fact that she is highly religious will be a dealbreaker for me, since I'm agnostics and our differences will surely clash. Her being religious = "Not my style"
"Not my style" is just a slang way of saying "They don't fit my preference".1
Most Helpful Girl
You just don't like them.0