First of all, Loras beat the Mountain in a joust, not a swordfight. The Mountain would tear him apart.
Secondly, he cheated. He used a mare in heat that made The Mountain's horse lose its focus.
Thirdly, when was Loras claimed as one of the greatest blades of Westeros?
All I saw was a giant woman beating a scrawny man, which sort of happens usually.