It's just so hard to believe it'd be all for nothing. The ocean theme is ridculously strong with her, the "natural color" hair stripe, Mermaid Star wearing the same goddamn shell necklace, everything.
I know Nefcy explicitly said she's not a mermaid, but maybe she's still got a distant merperson relative. Like she herself isn't a full mermaid but she's got some merperson heritage? A quarter-mermaid? Or perhaps she'll strike a close friendship with the seaple and bring them in as allies to the Butterflies?
Or maybe she'll get turned into one of the merpeople via magical means and it's all foreshadowing of her destiny?
I just, in a show like this, where even the $650 turned out to have an in-universe explanation that was a callback to an earlier episode, Jackie's consistent, blatant ocean theme can't just be nothing. I think Nefcy's just being evasive because she doesn't wanna spoil whatever it is they'll do with Jackie.