If your cat is losing hair on the back of ears, the first question that you should ask yourself is whether there is genuine hair loss because the hair on the back of a cat’s ear flaps (pinnae) is very thin anyway. It can look as if there has been some hair loss. This is why the ears feel warm because the blood vessels are near the surface as there is a very thin layer of insulating fur.
Secondly, although I am not a veterinarian (but have lots of experience), it would be unlikely in my opinion if there was hair loss at that location and nothing more. What I am referring to is such issues as the skin being abraded by scratching because the ears are itchy. This would cause hair loss but there would also be signs of scratching or overgrooming.
Nearly all hair loss on a domestic cat ultimately goes back to overgrooming or excessive grooming which is caused by the skin being itchy or the cat is feeling stressed. You have, therefore, to assess the underlying cause of the itchiness or stress. A classic cause of itchiness is the ubiquitous ear mite. Although it lives in the ear canal it can migrate out of that location to places around the ear causing irritation there.
Allergies and dermatitis cause itchiness and a cat may groom excessively in other parts of the body including the ears to alleviate the irritation. But as stated it is unlikely that you will see simple hair loss as the skin may be damaged by the cat self-mutilating.
White cats can get sunburned ear flaps which can cause the ears to become red and itchy. In addition, exposure to sunlight can lead to skin diseases in cats particularly white cats and the inflammation may predispose the cat to squamous cell sarcoma in those areas accompanied by hair loss.
Feline endocrine alopecia is a form of balding possibly caused by hormone deficiency but may be caused by compulsive self-grooming due to stress. Normally this condition results in a symmetrical pattern in the lower part of the cat’s body, on the insides of their legs and on their belly for example. However, it is possible that the cat may overgrooming their ears as well causing hair loss. Stress is normally caused by unwelcome circumstances in the cat’s environment such as long term noise, constant disturbances, hostile and bullying cats, strange people and so on.
My conclusion as a non-veterinarian is that you should first make sure that what you’re looking at is a normal amount of hair at that location, but if you are convinced there is hair loss look for the underlying reason for excessive grooming due to itchiness or stress because self grooming is a calming process.
Obviously, if in doubt see a vet about because that’s what they are there for.