If shes not directly touching it I cant see any risk of it burning her.
I'm not sure how well the heat will get to the cage though to be honest if its the top out of three blankets. But you can try it anyway, my only question is if you've got the blanket over her cage all day do you have a light source on a timer in there ?? As your blocking the day light with the blankets, although if its keeping the cage warm then it wont be a good idea to take them off in the day time, so having a light on the inside of cage on a timer will keep her day/night cycle better.