It's not only her self-serving tendencies, is her overall attitude with basically everything and everyone in Ponyville that goes beyond season 7.
Starlight basically encouraged Twilight to reopen her new school that was shut down for valid reasons.
She told Twilight in a condescending way that she's not a princess-princess.
She blasted Discord to kingdom come for heckling her.
Despite considering Trixie one of her best friends, she traded away her home without her consent and gets angry at her for being upset about it.
And speaking about Trixie, despite considering her and Sunburst her two best friends, she constantly ignored not only Trixie, but also her goal to properly prepare a party for Sunburst.
Starlight's character is fundamentally toxic and unchanged by the end of the show, because she wants to do whatever she wants and wants to avoid any kind of responsability. And that's a horrible thing to do.
Many people would point out this attitude if she was Cozy Glow or another villain, because antagonists tend to have this behavior in kids shows. In a show where she's supposed to be one of the main characters, she's horrible. She's at the same level as Superman from Man of Steel or Leonard from The Big Bang Theory: Someone who doesn't question her ultimate decisions.