Most of the revealing gear in the game, including the bikinis, are level 1 items. If a player chooses to use them for glamour, it has no effect on game play whatsoever; glamours are purely for aesthetics. Final Fantasy XIV has a pretty good mix of revealing and non-revealing gear choices to select from. In Dragoon's case, most of the higher level gear tends to be non-revealing, with a few exceptions. You will be able to glamour any piece of gear onto your current armor, as long as it is not above your current level, and Dragoon can natively equip the piece of gear you are using for the glamour.

So yes, you can avoid running around "looking like He-Man the Barbarian," once you reach 50. As for whether players running around using bikinis for glamour is a regular occurrence; it can be. It really depends on what a player feels like glamouring. However, you do not have to follow this trend, and you will be able to glamour your character with a full set of non-revealing glamour if He-Man is not your thing.