I'm rewatching She-Ra again (following along with Mortius who is ADORABLE, go throw money plz) and on this watch through I noticed something: Frosta's personality shift actually makes completely perfect sense.
Frosta is going through a lot when we first meet her. Her position of authority over her kingdom is important, and she's very aware of this. She's also very aware of how dangerous the world is, and has a VERY PERSONAL REASON to distrust these Princess Unification efforts that are going on. She doesn't have parents.
So going into the Princess Ball she has two goals, and they're a kind of important that would be a crushing weight to anyone, to say nothing of a precocious 11-year-old:
One: maintain. NORMALCY. The ball must project strength above all else. All eyes are on the Kingdom of Snows, and their image of being above the fray must not waver for a moment. Frosta is putting on an act in this scene, more than Adora. Adora is trying to be competent in a complicated environment. Frosta is trying to convince the world that her kingdom is not one iota weaker for having her as its queen.
Two: Leave ZERO room for the princess alliance to think they can peel off members of her court. We don't see Frosta's Wicked Visier in the back corner anywhere, because she presumably has a nice dull one, and we've seen that she cares about the plan enough that she may have even fired a few.
Frosta, as we first meet her? Holding on to an isolationist framework with an iron grip, holding her breath the whole time, refusing to budge an inch, because she must. Not. FAIL.
Then the Hoard destroys goal one entirely and forces her to rethink everything about goal two. But the thing is those weren't just goals for the ball, those were her whole organizing principle. That was her PLAN.
She doesn't get a lot of voice lines as she goes through two separate rounds of joining the rebellion in season 1, but something to keep in mind about this part of the story: Frosta has cast off most of her identity to do this thing.
Who IS she now? She's been a miniature adult since she was able to think complete thoughts. Always being guided towards her destiny, and her destiny was to PRTOECT. THE KINGDOM. ALONE.
But... friends? Other PEOPLE? People who... sort of UNDERSTAND HER???
And, my god, Princess Glimmer of Brightmoon is everything she WISHES she could be. Taller, confident, REMEMBERS HAVING PARENTS, has an identity of her own, NOT A QUEEN 20 YEARS TOO EARLY, able to have like, the absence of responsibility in any direction at all???
The Frosta we met at the ball was brittle. About to crack, really. And when she did, she lost her grip on her illusion of being an adult. She's a CHILD, and the princess alliance gives her enough breathing room, enough of a sense of not being the center of power, that she has a sliver of permission to actually BE one.
One of the big themes in She-Ra is institutional evil, and in the Hoard we see this tangle of miseries, thrown into even sharper relief by how good the people of Brightmoon are to one another. And the biggest misery, the one the whole show orbits, is the misery of being a weaponized child. It's why Catra and Adora suffer so much. It's the core of the story.
But what's the opposite of that?
Well, it's an eleven-year-old girl being allowed to fumble friendships with The Big Kids, indulge in her power fantasies, make up silly games from nothing and focus on her own chaotic, joyful energy. Frosta's change isn't merely explainable, it's important to the story. The show let her be a kid again, because it had to tell us what Adora and Catra should have been allowed to have.
Also I think her precociousness might have taken a bit of a narrative back seat, but we do see it again, especially near the end of season 4 where she has that absolutely blockbuster line "they're people." Frosta can seem like kind of a lot in a story with so many heavy themes, but she's a necessary kind of a lot. Because She-Ra is about building a world around kindness, about the awful cost of not doing so, but also, about the rewards of doing the hard work to do it.
And it is both part of the hard work, and a reward, that a kid is allowed to actually be a kid, even when the people around her have to pick up some of the slack left when she doesn't sacrifice her childhood for them.