A lot of people have been using search terms relating to the baldness of cats ears and ended up here.
My cat went bald about the ears for a while. I thought it was old age, as there were no signs of infestation, skin infection or anything else. Then, just as mysteriously as the hair had disappeared, it reappeared. Weird. No more bald-eared cat. I still can’t work out what happened.
So, if your cat has bald ears, and you can’t see any signs of anything wrong, then it may pass. Do check with your vet though if you have any reason for concern.