That and her lackluster character to begin with.
I don't think there's legitimate hatred for her, just glaring indifference on one end (namely from those who value Starco and the story more, understandably), and frustration on the other from people who want to see more of Jackie in a meaningful manner.
Though I have to say, I never see so much backlash from other people when a character whose appeal is based on potential doesn't end up meeting that potential. Why do Jackiefags invest so much in what she does?
The fact that so many just want something for her, regardless of whether she's a mermaid, or a villain, or comes with Marco, or stays on Earth, or remains a friend or not, leads me to believe most people just like her because she's hot and they want someone pretty to look at.