Personally, I'm not really buying the whole "Eclipsa wasn't really evil/was just misunderstood" thing that some fans believe. Based on what we've seen, do you think she could be an example of Affably Evil or the reverse of Good Is Not Soft ("Evil Is Not Hard")?
I don’t know for sure. Eclipsa is certainly more than what she seems, and the show is setting her up for a big reveal. Personally I think she’s not as evil as people in the show say she is. She’s feared because she mastered dark magic and ran off with a monster lover. In fact Monster Lover is one of her titles, and we’re already seen that Mewni’s monsters aren’t pure evil. The Mewmans invaded their home and they’ve been oppressed, feared, and hated for years. So for a Mewman queen like Eclipsa to be sympathetic toward them and even fall in love with one would be a huge scandal and make her a target of fear and scorn.
We also don’t know the reason why Rhombulus put her in the crystal. Did she actually do something wrong or was he acting on one of his whims? He obviously thinks she’s evil but his word is unreliable since he’s zapped people who did nothing wrong before. Glossaryck spoke positively about her, saying she was the only queen who let him be. And Glossaryck is confusing and has his own weird set of morals, but he’s usually right. So Eclipsa sounds like she could be a complex character, someone who lives by her own rules and does things her own way for her own reasons that could be dangerous and may or may not be socially acceptable. Not unlike Star Butterfly herself.
Also we still don’t know very much about dark magic in general. Is all of it dangerous and does it always lead to corruption? Star looked at Eclipsa’s chapter before and nothing bad happened to her, but it had a powerful effect on Marco. Why is that? Is dark magic less stable and more unpredictable? Are certain people more susceptible to harm from it? Does it take a special kind of person to master it?
And then there’s the possibility that Eclipsa had children. She’s a Butterfly but could she be related to Toffee somehow? Is Toffee half-Mewman? Or what about Miss Heinous? She has Butterfly family cheek marks but nobody seems to know about her connection to them. Who were her parents? Is she a long lost Butterfly through Eclipsa’s bloodline?
So yeah, I don’t have a solid opinion on Eclipsa yet. I just have a bunch of fragmented ideas and theories floating around my head.