A depressing Just Friends theory: the part Star and Marco lipsync to is about having had, and wasting a chance to be more than just friends; that coupled with how reluctant Marco was to have Jackie come with them and her lack of appearances. What if Marco was getting over Jackie (possibly due to Running With Scissors) and Star unwittingly rekindled Marco's feelings for Jackie, thus wasting her chance?
I don’t think that’s the case, to me it looks like Marco was mostly worried about Star monopolizing Jackie’s attention: remember that, to Marco, Star is the coolest girl around; he said it in the first episodes, he agreed to it again in Just Friends
So to someone as insecure as he is, the idea of someone like Jackie, another cool girl, wanting to spend time with him over spending time with Jackie sounded absurd
So it wasn’t as much as Marco getting over Jackie, more like Marco still not feeling completely comfortable around Jackie, still worried about losing her over the slightest of mess ups.