Yes, cat ears are supposed to be hot for the simple reason that the blood vessels in the ear flaps are very near the surface and there is very little hair or fat between a person’s touch and the ear. Therefore, you are almost feeling the body temperature of the cat via the blood. The body temperature of a cat is between 99.5 and 102.5°F.
Just to be clear, when we are talking about “cat ears” in this instance we are talking about the visible part: the ear flaps or pinnae. There is a lot more to the cat ear than that. In outline, there is the flap, the canal, the eardrum, the middle ear and the inner ear.
It is interesting to note that nowadays when a doctor checks the temperature of her human patient she will sometimes do it with a device which is inserted into the ear. The temperature is measured very quickly and accurately. This reinforces the idea that the internal body temperature of a human or a cat is very near that which we can touch in or on the ear. This is why it feels warm or in the words of the title to this article ‘hot’. I would not, to be fair, described cat ears as hot. A more accurate description would be warm.
In warm weather the ears will be warmer and if a cat has been very active they may feel warmer. Cat’s ears are vulnerable to sunburn and frostbite in because the protective covering of fur is very slight. Cats with pink skin are particularly vulnerable. The ear flaps are also vulnerable to damage in fights which is why cats fold them back prior to fighting (also a body language signal).
What I have written does not preclude the possibility that your cat genuinely has a temperature or a fever. I’m just stating the obvious that the ear will feel warm but this might hide a genuine illness, one symptom of which is a high temperature. If in doubt please see a veterinarian.