Depends on what the promotions allows. They usually have strict rules on what you can and cant wear. Usually they have a list of what is approved. And I would be willin to bet a mankini is not allowed.
Honestly... the extra oh i dunno maybe pound from shorts wont hurt you at all. you train with a shit load more weight on from pads/clothing as it is.
Isshinryu Karate 2nd degree Black belt.
Neko-Ryu Goshin Jitsu 2nd degree Black belt.
Jujitsu 2nd degree Black Belt.
Isshin-Jujitsu 2nd degree Black belt.