It makes her more relatable and also makes us cheer for her harder.
When Star screws up, it's usually her own doing, and she oftentimes pays the price, hard. With Marco, the wand, etc.
But these mistakes also humanize her and we know that Star is still a pretty good and nice person so we still root for her to make it despite the adversity she goes through (which is largely her own fault).
Having a perfect/near-perfect MC who either doesn't screw up, or screws up and immediately fixes things isn't BAD, but it's tougher to relate to. Having a MC who screws up because of her own personal failings/flaws, gets screwed even when she tries really hard to do the "right" thing (setting up Marco and Jackie), and struggle HARD in spite of all those things, with a serious chance that things don't work out?
It makes us root for her happiness harder. We know it's not guaranteed, hell, so far, Star's suffering is the only thing really guaranteed. So we want her to succeed and become a better person so that when she does get what she wants, we feel that the journey really mattered