CORTINA D’AMPEZZO, Italy — The Wonder Woman suit. The red cape with a big “J” printed on it. The plastic tiara. Julia Mancuso’s outlandish costume for her retirement ski race was a reminder of what made her so great: she was a free-spirited Californian who consistently beat the favorites when the stakes were the highest — just like a superhero. Mancuso, the most successful female American skier at major championships with nine medals spread between Olympics and world championships, bid the sport an emotional goodbye in a World Cup downhill on Friday.