I'd argue Captain Syrup was originally intended to be the "Wario" for Princess Peach. One is the ruler of the land, the other is the ruler of the seas. Peach, as a fruit, is good for you, meanwhile Syrup, and by extension her crew being called the Black Sugar Gang, are bad for your health. Peach is "a love interest" while Syrup is a rival. She even has long hair like Peach so very much an evil Peach recolour. Plus it's very apt that Wario's games would involve seeking out his "Peach", but not because she was kidnapped, but rather because she kidnapped something he was after.
Heck, we've seen another princess/pirate connection in another Nintendo series with Zelda and Tetra being two sides of the same coin.