The ears should feel cooler than the rest of her. In part, the ears help to keep the dog's body temperature constant by allowing the blood to cool as it passes through the vascular system that is very close to the skin surface in the ears. This is important because dogs don't perspire the way people do to cool their bodies. If her ears are ever very warm to the touch there is a good chance she is running a fever and might be sick. Good for you for noticing this and getting an answer to your question!
The dog's ears could be due to an ear infection that causes only the skin of the ears to feel cold. However, if the ear is not red or removes a bad smell, then it cannot be a serious problem. You should take it to a vet for the right diagnosis and treatment.