Depends on the context. One-sided interest that's fixated on one particular characteristic comes across as creepy - it's obvious when it's a physical characteristic, but it's also just as creepy to be particularly interested in someone's career, or house, or car, or any other thing. One-sided interest that's diffuse tends to become two-sided interest, because it makes you seem engaged with the world, and that's a very attractive quality for a lot of people. Basically, be interested in everyone and you'll find that everyone is interested in you, while if you're interested in only one person she probably won't be interested in you.
