Poor kitty! Good for you for caring for her.
I don't know if you can use that on her. I'd give your vet a call. But, if she has really bad mats, that can't be combed out, probably having her shaved would be the best way to give her relief. Mats can cause serious discomfort and, as you know, badly irritate the skin. I would not attempt it yourself because they can pull the skin up and, unless you really know what you're doing, you could cut her.
Hope that helps.