"everyone but Jackie" meaning all the other teenage girls, yeah.
I don't think they're all necessary meant to be flat, rather I'd say Jackie is meant to be curvy. She has a very distinct character design that, I assume, is meant to clearly draw attention to her attractive features, with her being uniquely curvy among the girls at school and even having extra detail in her eyes. She's given the extra attention because she's Marco's crush.
Even other girls you'd expect to be attractive like the cheerleading squad keep the same body type.