I wouldn't say we can be too sure about the state of Jackie and Marco's relationship just yet: we haven't really seen all that much of it. We've never had an episode where Jackie's risen above tritagonist rank, and the only episode where she was possibly more important than Star was "Naysaya" (and there she got bumped out of the deuteragonist slot by a neck man!) and in every Jackie episode since then, Star's reaction to Jarco has gotten at least as much focus as the relationship itself.
Even though I'm on the Starco side, I WOULD like to see more of it; I would like to see them actually happy together, if primarily to boost the emotional torture of Star. But on that topic, I've got to say I'm really glad that Jackie's turned out as great as she is. My initial expectation, way back when, was that because Marco admired her so much, she'd turn out to be the opposite of deserving; probably a "don't talk to me, loser!" type. And I'm glad that her relationship with Marco hasn't been treated as a bad thing; an obstacle for Star to fight. That would be too easy! Instead, she's had to make a hard choice: whether to suppress her feelings and hurt herself, or let them out and probably hurt Marco (and thus also herself!)
Now Marco's got to make a hard choice, too. What does he really want? He can stay with Jackie, on Earth, if he wants a normal summer and a normal relationship with a normal girl. It's not a bad life; it's what his parents have. Or does he want things a bit more weird and a bit more wild, with Star and all the (literal!) magic she brings to his life? Thing is, they're both worthy choices. So it's crunch time for Marco now!