You know that's a bit of the problem, don't you? (Personal interpretation) 1) A lot of the things Uta does, especially things like transforming her fans into objects or summoning Tor musica, feel like childish tantrums. A lot of her dialogue and attitudes make her seem incredibly immature and stupid, that many of those actions are done at times when the character is treated with the patience of the world by everyone just makes her look worse. This is bad for an antagonist, because villains are supposed to be obstacles to heroes and Uta's basic motivation as an ambassador for civilians had some merit and logic to it. But nothing real was ever done with it, Luffy or Shanks never reacted or questioned whether Uta was in the right. Uta does what she does in the end because of her trauma, narcissism and because she wants to kill herself, her actions, lies and plans are little more than a way to justify her suicide and anger. Her hatred of Shanks? A lie and tantrum to justify herself, even in the bonus material she says she wants to see him again. Save her fans, hate pirates and make them happy? An excuse to justify herself and take them to a world where they can worship her eternally, because her paradise was that, a world where everyone will live to hear her sing and worship her as a goddess and saviour. Summon Tor Musica? Uta doesn't care about anything anymore, she just wants to be left alone and prefers everyone to die rather than acknowledge her mistakes, her ideals were excuses in the end. That's why Uta is perceived as a tantrum child and a terrible person that half the world loves because she sings beautifully and the rest loves her because she is a friend of the protagonist and the protagonist's mentor.
2) Uta's past/history is pretty poorly written on a plot coherence level, there is literally no reason for her to have stayed 12 years in Elegia or for the island not to have been repopulated. Everything about Tor Musica is incoherent within the logic of the movie itself, if Gordon knows there is a demon on his island that can only be awakened by Uta, why didn't he warn Shanks and Uta about that risk? If you add that her ignorance feels weird considering she's the daughter of a world-travelling pirate, they live in a world where the newspaper literally flies in and she has access to basically the internet, where even pirates are her fans, her ignorance is kind of forced. That includes her sacrifice, Uta after freeing Luffy and company could have taken the antidote perfectly well, she was leaning on Shanks for a while. Even Shanks could have used his haki to knock Uta out or at least try to. If you add to this that Uta's original plan is to die, reconcile with her father and be remembered as a saviour, it feels more like a reward for the character than a punishment. 3). everything about the mushroom feels cheap overall, we know too little about how Uta acts without the mushroom to be able to differentiate her actions from what is basically mind control and how the character acts very calm in her worst moments doesn't help, she doesn't have dark circles under her eyes, she dances and sings as if nothing is wrong with her or any kind of discomfort until very late in the film.

By this I'm trying to tell you that while Uta has motivations, the kind of villain she is feels very basic and cheap, the only way her actions make sense is if you see her as a tantruming child, who doesn't know what she's doing, who also has mind control, who to top it all off her problems are easily solved for the viewer because they could have been solved with literal talk or common sense.