Based on the Übermensch or its rough translation to superman from the writings of Friedrich Nietzsche, the Ubercunt is a super cunt. The Ubercunt is a woman who has become so incredibly bitchy, mean spirited and evil that she's evolved beyond a mere cunt.
The ultimately cute person. The most attractive person imaginable. Someone who is as attractive as they possibly could be. Someone who is not just physically appealing, but is also intelligent, fun, funny, vivacious, and friendly.