The Mewni map shows the coast is a dump, making mermaid emigration plausible on top of the mewman vs. monster thing.
Jackie is never seen without a seashell necklace which could be deemed a magical amulet that lets her have human form. If not that, then a personal reminder of her ocean origins. See also the seemingly sea-themed dress she wore to the dance.
The streak in Jackie's hair is natural.
Mermaids are always beautiful which would lend a reason to how she's by far the hottest girl in the Earth setting. That'd be a cool trick; her beauty was obvious from the first time the audience sees her but they fail to realize the implications. It'd be a cool "Oh, I see!" moment.
Having Jackie's family being monsters who crossed dimensions to settle on Earth merges the mundane and fantastic aspects of the setting in an interesting way. Marco realizing he grew up with, and fell in love with, a monster (in Mewni terms) alone has interesting implications. After a few seasons the divide between the two elements of the story isn't very engaging anyway.
But they decided to do nothing.