I try to give every merfolk kingdom in my universe a unique culture, even though it can end up being the "planet of hats" trope. BellaSea is beauty-obsessed. Atlantis, which is featured in A Tail of Two Mermaids and A Baker's Tail, is socially progressive, high-tech, and sort of retro-futuristic, with '90s-style undersea shopping malls, and music played on SeaDiscs in Swimman devices. I'm a CD collector nostalgic for '80s/'90s mall culture, which is a vibe I never see in traditional high fantasy. Baltica is small, low-tech, and old-fashioned, basically the merfolk equivalent of a quaint small town in a Hallmark movie. Reddia, from The Sea Princess and the Storyteller, is Middle-Eastern coded. Pugetia, which appears in Pearlina and the Water Faery and The Siren Queen, is a quasi-dystopia in which most of the population lives in poverty while the royals live in luxury, satirizing wealth inequality in my home region of the Pacific Northwest. Pacifica is probably the least unique, just a pleasant kingdom with benevolent monarchs.
