First, they've had like 20 minutes of shared screen time throughout the series. Apart from kissing, Jackie has barely shown any Affectionate behavior with Marco that she hadn't shown other character . Certainly nothing close to how he and Star treat each other.
Secondly, all of their moments are the same moment. Marco is nervous, Jackie is comforting. That is the absolute extent of their relationship so far. Jackie hasn't laughed with Marco, she hasn't cried with Marco, she hasn't just sat around bored with Marco. The most upset we've seen her is when they got to the school dance and it turned out to be lame, and she addressed that with the same monotonous cali girl drone she carries in her normal speech. Beyond being "chill" she hasn't developed any more personality.
Finally, you can put any two characters in generic, cutesy date scenarios and make their relationship look healthy and perfect. I could draw Morticia Adam's making good good eyes at Johnny Bravo at a coffee shop and they would look adorable despite the scenario not being even slightly indicative of their characters.
Apart from the skateboard scene, Jarco has been a lot of safe, cutesy, generic tripe.