Yea, I don't mean Star is totally awful and should just die or whatever, and like I said I do think the big arc is her finally learning and growing into a better person, she clearly has the makings of one. I do think she's currently a bit of bitch, but sorta unintentionally - she wants to be and perceives herself as a nice person, she's just really, really spoiled and used to getting her way, and isn't used to evaluating her own feelings, actions and choices critically.
I just thought about how the song day illustrates that further; Tom and Marco point out for example that she's used to changing the rules mid-game so she'll win, because she can't deal with losing even a ping pong game, she's used to bending the rules and getting her way without actually earning it. Now, she's aware she "stars" things up all the time and doesn't measure up to the responsibility of the crown, knows Moon is disappointed in her etc, all that. It gives her terrible angst and insecurity.
However, her response is not to put in the effort to improve, but instead once again to try to change the rules so she'll win as she is; instead of delivering a trad. Princess song and working to become a trad. Princess /Queen, she delivers a song letting everyone know she's a fuckup, deal with it Mewni this is the real me, I won't be a perfect Queen so get ready.
She tries to lower the bar of expectations to where she is, instead of working on improving herself to meet the expectations.
Contrast this with Marco, for example, who works to improve and when facing a daunting situation where he just doesn't measure up (Hekapoo's challenge), he just instantly grits his teeth and gets to work on improving himself and learning the skills, till he DOES measure no matter how long it takes.
(I think this might be used to boost Starco, as this very important attitude and maybe teaching it to Star is something Marco can bring to the table)