NTA. I’m gonna go against everybody here, but I want to clarify I don’t think you’re the asshole. The sitter should absolutely not be paid considering the damage of your property, but I think you’re responsible here for not clarifying explicitly about the extent of cleaning the litter box (I assume you didn’t).

Yes the sitter should’ve followed all instructions. However, the sitter probably thought that they could get away with cleaning it every couple of days rather than every day, and didn’t actually think that the cat would deem it unusable. If they knew about it, it’s a different story though. But I do think you should’ve made it explicit, outlining the effects of what would happen if they didn’t clean it everyday. That said, NTA ultimately and she doesn’t deserve pay due to the damage.