Looks are almost never the real issue. You have to try and figure out why guys don't think you are approachable. Here's a few ideas of mine
1 You are always with other girls? A pack animal is terrifying to approach!
2 Your too nice and maybe feel like a sister or mom?
3 You just haven't been around the right guys enough to get noticed?
Not saying any of these is the case because I don't know you. Would help if we knew more about your personality and your habits such as when you see guys and etc. if your still worried about looks, post a picture!
No it does not make you ugly. It means that you have not found a guy who likes you. Even with adults we need to go looking around for people that want to date us. It is a complament to your character that you are being sister zoned and friend zoned. It often has nothing to do with your appearance. But with the kind of guys your around. Also like nw5394 said there are a lot of other factors that contribute to not getting asked out.