Bodacious Battling Bikini Babes – Do they ever make sense?
No, is the short answer. At least not in any of the fantasy settings we usually see them in.
Historically of course there are cultures that went into battle wearing very little, such as Zulu warriors, although they didn’t go into battle completely unprotected – they carried shields (and still do in fact, but not really to war anymore). There are Roman accounts of Celts going into battle completely butt naked, although that wasn’t the norm. Typically though once a society has the resources and technology to manufacture armor warriors will choose that over nothing at all. Otherwise their enemies will wear armor and thus will win.
That’s the big problem with Bikini warriors – you see them in worlds alongside or even in the same society as other warriors who do wear proper armor. Take the Amazons in Justice League – they clearly have the technology so there’s really no excuse for them ever to be dressed in a leather bikini, at least not when guarding or going into battle. Maybe for training or athletic contests – obviously athletes wear as little as possible for perspiration, but in battle the benefits of solid protection outweigh of being able to go on just a little bit longer (not going to go on long at all if an arrow tears through your exposed belly anyway).
Maybe you could have a culture or religion that encouraged it’s people to dress down for a fight, but they just wouldn’t last long if they’re up against full plate and steel.