Having Mina as the main villain is shit for a number of reasons:
Firstly, Mina isn't some bigot who just hates monsters for no good reason; she's was a volunteer soldier who only wanted to protect her people at a time when Mewnians faced a very real thread of extinction.
Secondly, Solaria's spell explicitly screwed with her warriors' mind; making them incapable of feeling remorse or empathy. Mina is literally incapable to recognizing that what she's doing is wrong.
And Thirdly and most importantly: The analouge of Monsters as a powerless and discriminated minority (the obvious parrallell being minorities in today's America) that the show is trying to pull doesn't work; it has been established that the monsters are both capable and willing to threaten the very existance of the Mewnians and that magic, and by extensions the Solarians, are pretty much the only reason they haven't done so long ago.
Just depowering Mina and have her stomp off into the woods was not only an unsatisfying end to her character, it also does nothing to address the still very present racial tensions. No dialogue, no attempt to work towards lasting change, just "huh, guess you just can't reason with some people".
Aknowledging that there is more to her than just being a biggot and making the effort to free her of Solaria's spell would be a far more fitting ending, and honestly a far better message to send.