As long as shes on top of it it's ok. It should be zipped closed to ensure she doesn't try to climb inside. The vinyl on these things is pretty stiff so it wouldn't hurt her unless she got inside of it.
LOL...reminds me of when you were a kid, and at some of your friends houses, the mother would have all the living room furniture covered in thick plastic, and there would be plastic runners on the carpet that you weren't allowed to step off of!