While I see your point of view, I must respectfully disagree. While Star's jealousy is portrayed as a negative thing, her trying to suppress it and help Marco with Jackie is ALSO portrayed as a bad thing. She's being dishonest with herself, because she can't bring herself to harm Marco and his relationship by confessing. Star isn't acting spoiled and trying to fuck over Jarco, if anything she's been doing her best to support it, and has even tried to befriend Jackie as well. Not only that, but it's also a matter of character development for Marco as well. He's put Jackie on a pedestal for a long time, and supporting such a thing would be just as bad. After three dates, neither we nor Marco know much about her other then what's on the surface. And what's on the surface reinforces the pedestal he put her on. With Star, Marco knows her strengths and weaknesses, and has seen her at her best and at her worst. For someone like Marco, he's able to connect more with the girl who's shown she's not perfect rather then the seemingly perfect Jackie.
However, this is simply my opinion. No matter what happens, I hope you enjoy the show, and where it goes with the story.