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She actually suffers for her mistakes is the thing and that makes when she can be a bit annoying acceptable.
For example, compare her to say, Mabel Pines
Mabel got away with everything, Mabel got everything she wanted and Dipper had to eat shit on a sandwich if it meant Mabel not suffering, she was obviously in the wrong and yet reality warped itself around her to make her right. Oh and of course she shit all over Dipper when he made any judgement calls because obviously they were wrong.
Star meanwhile is kind of the opposite, she's incredibly supportive of Marco, even when it hurts her, she doesn't get what she wants, she's been handed very little plot wise and had a lot taken from her. In fact she's given more to Marco than shes taken and put his interests above her own to make him happy.
That makes her incredibly likeable as a character. The fact that even though she can be a bit annoying or dense at times or make the wrong choice, she's trying to do the best for those around her and make them happy, even at the cost of her own happiness. When she stumbles (See the spying spell) no blows are pulled on how it hurts her or that she has made a mistake.
And her personality/attitude is the cause of the majority of her problems.
Basically she's the Cartoon equivalent of Elric of Melniboné (In a hot pink dress), well meaning but poor in judgement, started with everything and it is slowly being whittled down by her own human flaws and bad calls.
