When I was younger I used to be very ugly, but I cut my hair short, got contact lenses instead of glasses, started going to the gym and wearing better clothes, and I'm quite good-looking now. I think a lot of people who think they're ugly could probably be a lot better looking if they did the right things. I'm not saying everybody could be a model, but someone who's a 2/10 could probably make themselves a 6/10 for example, unless you've got 3 eyes or your nose is on the side of your head or something.
I only have phases where I think I do, or when I really think about whether I am or not for too long.. I think that admitting that you feel that you're ugly makes you unattractive whether you truly are or not.. Confidence can sometimes be more attractive than the looks themselves..