@r-m-archer absolutely! Captain America often underwent flat arcs, and I think his movies are some of the most powerful Marvel movies.
In addition to changing other characters, this MCs of yours will probably be changing the world around her as well. Like a man vs. world sort of thing, or she’ll be fighting against a certain ideal. It can make for a really powerful story.
Another example is Batman Begins. Though Bruce undergoes a mini-positive arc in the first half of the story, he’s essentially carrying a flat arc through the rest of the movie.
As a final thought, I don’t think a character has to be necessarily perfect or noble to have a flat arc. They can have a flat arc with a negative belief as well. It’s just the process of the character changing those and the world around him that makes it a flat arc 🙂
"Beauty will save the world." - Dostoevsky