There are other super heroines besides Supergirl that have major super strength. Wonder Woman, Power Girl, Rogue from the X-Men (though that's left out in the movies), Spider Woman, and more.
As for why they look like super-sexy bimbos... That is because, like other people have said, they are created by and drawn by men for men.
That being said, when you look it from a more objective viewpoint, the male characters are also drawn to look super-attractive in their own way. True, the female characters all have huge boobs, hot bodies, are generally good looking, and wear costumes that exploit all of these traits. But look at the male characters- They are all tall and handsome, with perfect muscular bodies and spandex costumes that show off their perfect muscular bodies.
So if a little girl is looking at comic books, she might get frustrated by her inability to look as good as the super heroines. But little boys might feel just as frustrated that they don't look at good as the super heroes.