You can use hydrocortisone cream on a cat but you just want to make sure that he/she doesn't consume much of it. ALso, depending on the type of lesion you are using it on unless you know it is due to inflammation related to allergies etc, just be very careful. The reason being is if you put it on something such as ringworm etc, it could make the lesion worse. So just use it with caution and use your best judgment if it isn't getting better soon.
Also, apply the cream and then distract your cat so that it has time to soak in and work.
I hope that helps. Let me know if you have any other questions.