Exploring "dark magic" doesn't necessarily make her the Wicked Witch of Mewni, and it's not like all of her spells are inherently "dark" or evil. The All Seeing Eye obviously has terrible potential to be abused, but when cast it's an ordinary "good" pink spell that doesn't do any harm on its own, and it's Star's own jealousy and lack of self-control that gets Marco and Jackie hurt.
Star herself read through all of Eclipsa's chapter, and while she describes it as "power of darkness, forces of evil, eternal suffering, blah blah blah," she certainly didn't react as if there was anything especially awful in there, like rituals to sacrifice children.
Glossaryck, who is the closest thing we have to an impartial reference, at least spoke favorably of her to Queen Moon and wasn't nearly as worried about Star reading her chapter as the rest of the High Commission.
All that said, she obviously ended up as some sort of villainess, but given that her most terrible crime was apparently running away for true love with a monster in an apartheid society, she's clearly being set up as tragic at worst. As opposed to Toffee, who's gotten several episodes of screentime, set himself up as the BBEG, and is still an unrepentant villain with zero redeeming qualities.