It's easy to choose the right thing to do when the one proposing it is polite and charming. It doesn't require any thought at all. "Yeah, sure fine, whatever."
By forcing you to balance your dislike for the character with your agreement with her goals, it makes the choice harder. It makes you stop and think what you really want to do. It makes the choice harder, and that makes you more invested in the decision than you would be if Catherine was nice and you wanted to please her.
Does she get better? Well, if you finish her arc with mostly decisions that agree with you, then she will have words of praise for you.
