In my personal experience Cassie doesn't lose Match-ups, she just has some pretty bad 5-5s. Having a 9f mid into decent damage, proper zoning and some pretty good footsie tools, I feel like she's simply not as easy to play as the top 5, but she's right outside that.
Again, though, my guess is as good as yours. I haven't spent an enormous amount of time actually getting in there with her and grinding out each and every Match-up to a T. I feel like she's very strong and will remain to be, especially when the offenders of the most Overperforming moves will be taken down a notch.