Mermaids are not universally welcome at human balls. In most kingdoms near the sea their presence is unremarkable, but the further inland one goes the less mermaids visit, and the appearance of one at a ball can be a serious disruption. It’s not surprising that they cause annoyance and envy among the human women and prompt duels and inconvenient attachments among the human men, but besides that they are difficult to feed, not always aware of proper deportment and their air of superiority and condescension is often a little hard to take for humans of either sex. One insecure queen went so far as to ban them from all events given during her reign. (Mermaids mostly stay out of human politics, but stung by this, the equally insecure empress of the nearest mermaid empire ensured that said reign would be a short one by secretly inciting treason and eventually civil war in that kingdom. The loss of life and damage to the kingdom was incalculable, but the mermaids got their dances back.)
A mermaid choosing a gown for a ball thrown by humans generally wants to outdo every other woman there, human and mermaid alike, because the most common fault among them is vanity, followed closely by pride. Some of them do it by going with human fashions, thereby beating the human women at their own game, and some prefer to go with gowns designed for mermaids, which tend to evoke the sea, be less formal and hide the legs. (Most mermaids are self-conscious about having legs, as the vast majority of cheap mystics really don’t have the skill or knowledge of anatomy to form perfect ones for very long, so mermaid skirts are inevitably long and loose. The mermaid wearing a miniskirt is the one who gave up her firstborn.) This dress is definitely a mermaid gown; the human women at the ball where this will be worn will all be wearing more elaborate gowns, closer to what I think of as stereotypical princess gowns: tight bodices, poofy skirts. (Although some human women near the sea, where mermaids are more likely to show up to balls, have taken to wearing things mermaids can’t: shorter dresses, gowns slit up the side, tight skirts.) The choli-style top, the lotus and wave pattern, the fluttery aquamarine overskirt all make this gown arresting and otherworldly: just the thing for toying with the hearts of humans, leaving them crushed like a shellfish dropped onto a rock by a seagull. Later the human women will gossip about how revealing and tacky the top was, how unfashionable the whole savage getup was compared to their gowns, but the target of their ire will be already safely back under the sea with new stories to tell.