I see what she means. I took a look at the chipmunk fan fiction, I wasn't thrilled with how they portrayed Brittany. They always make her so mean and Nast to the point where I think, "That's not Brittany!" Bratette is the only one I know that captured their personalities.
I' happy for Alvin but at the same time, I feel sorry for Brittany. Remember some of the late episodes of the series where Alvin's been seen doing some good while Brittany's bring a little brat. I think that must be it.
Well, it doesn't help that Brittany and the girls don't get as much screen time as the boys, so you don't see as many dimensions to Brittany's character. It's actually been a while since I've seen the show, so I don't remember details.