Cats typically don’t breathe through their mouths. A cat’s normal breathing rhythm should be quite smooth with a normal rate being 15 to 30 breaths per minute on average.
So if your cat is panting it is best to take a look at why this might be the case and then figure out if you need to act.
They are excited
This is because they are learning to regulate their behaviour so may over exert themselves with excitement and then need to pant to calm down.
Outdoor cats in particular can use panting to cool down if it is hot outside.
Stress or Anxiety
Cats can pant when they visit the vet. This could be because they associate the vet with pain or because the way they have gotten to the vet is stressful.
They may have respiratory issues such as cat asthma or some form of bronchitis.
Heartworm in cats can cause panting and other breathing issues.
Some cat breeds are known to pant more often than others.