It just depends on your perspective as to whether you like her.
Jackie doesn't have any real character flaws as of yet. She has a chill/relaxed personality, she comforts/consoles Marco whenever he spergs out due to his insecurities/social anxieties instead of just reacting with disgust/hostility, she was perfectly fine with Star tagging along on her date with Marco when you'd expect most GFs to be jealous/pissed. She's a great girlfriend.
This makes her a boring character. The lack of character flaws means there's no room for character development, so she looks flat. Reacting sensibly/maturely means there's less chances for drama. It also makes her hard to relate to, since most IRL people have flaws of some sort, and generally don't like looking at people who don't have those flaws. This is exacerbated by the fact she's the heroine's love rival.
Naturally, people don't like the character. Not because she's done anything wrong, but because (1) she's not interesting from a work-of-fiction standpoint, and (2) she stands in the way of the heroine's romantic sub-plot.
But IRL, character flaws aren't a good thing. Real-life isn't fiction, dealing with real drama is a hassle, dealing with shitty people is much harder than watching shitty people be shitty. IRL Jackie would be a great person to be around. People as understanding/mature/relaxed as her are uncommon for a 14-year-old, but certainly not non-existent (e.g. I knew 2-3 back in high school).
That's part of what causes the divide here. Jackie story-wise is boring/hindering the plot, Jackie person-wise is great.