Is it normal for cats to pant hours after giving birth?

If she is no longer in labour and there are no more kittens to be born (cats will often have part of the litter, stop for several hours and then continue with the rest of the litter),  then panting could be a sign of infection. Mother and kittens should be checked over by a vet within 24 hours of birth so the vet can make sure the kittens are all ok and that the labour is complete and nothing left behind.  If mother has had water and food after giving birth and is still panting, then I'd recommend at least a phone call to your local vet.

All the best...

Of course. They have been through a painful ordeal & she still has healing to do. Dogs & cats will pant a lot of times when in pain even if its not hot.