She probably isn't. It's just that everyone is playing her. So tons of people have to lose to her. Same thing happened to Alpha, and then people stopped playing her. Same thing will happen to P-4 when she comes out too.
She's not broken, she's just new, and people aren't used to her. I mean technically, she COULD be broken, but no one's played her enough to know that, and at the very least, no one has found any broken tactics yet.
So she 'could' be broken? I don't really know how she could be broken though... eh. I'm just over people telling me I'm using "newb" and "broken" characters hah. Yet the people that are saying this are using the characters placed higher on the tier list on this site. The girl who messaged me uses only Sarah.