If you find someone to be unprepossessing, you find them unattractive. Not that they're ugly, mind you! Just unprepossessing.
Unprepossessing is a rather indirect way of calling someone unattractive, or at best OK-looking. Unprepossessing is not quite the same as "ugly." Rather, just a way of saying that someone's looks aren't what you're most likely to remember about them. Cinderella was most unprepossessing in the filthy clothes and worn-out shoes that her step-sisters forced her to wear. But when she was all decked out by her fairy godmother, she was the belle of the ball: she was no longer unprepossessing.