Millie Bobbie Brown was fine in King of The Monsters, when she was still a kid. Her undying faith in Godzilla was endearing and charming.
In Kong Vs Godzilla though, I found her to be kind of an annoying know-it-all teenager. She just seems like a frustrating conspiracy theorist, rather than someone who loves Godzilla.
That's a weird way to see her character. Her parents work at Monarch, so it's natural she has knowledge of the monsters. She was up close with Mothra. She was the only one to see that something is not right with Godzilla.