The first and most glaring reason is that she's just an annoying, and bland character, like most female leads in FF games.
During my run of the game however I felt that her stupidity was largely responsible for the destruction of Cleyra and Lindblum. I get it, she wanted to believe that her mom was a good person, but don't you think that when you hear how "Burmecia is under siege from Alexandria" your first impulse should be to go there and help repel the attack? Apparently you should just leave your friends, delay their response by poisoning their food and then go walk into a fucking trap, giving the enemy more power in process.
Then when she returns everyone is just cool with it instead of freaking out about the fact that she's partly responsible for the deaths of thousands of people. The fact that the story started focusing on her fucking dumb ass is why I stopped playing the game altogether.