Sub-Trope of Wearing a Flag on Your Head, where the flag design is on a swimsuit, or at least a similar garment. So while the larger trope is using flags to create a distinctive style (costume or otherwise), this is also done for outright Fanservice. Some people consider this disrespectful to these flags (and Brazil actually bans their flag on swimsuits), while other people don't seem to mind, as shown by the popularity of this kind of suit. It often shows up in advertising, particularly if being worn by a Hood Ornament Hottie.
Examples (Rolling Updates):