I think Gwendolyn's answer is making a great point.
I do want to point out though that you do want your cat to be ok with a carrier again.
The cat does not need to sleep in it of its own free will, but it should accept being in there as not terrible.
I had a cat which did not like the carrier and she would be afraid and anxious of being in there. This was alright since we rarely used it; just for vet visits.
The cat then developed a heart issue and being afraid in the carrier would cause panting and stressing to a much higher degree.
What I want to say is, follow the advice given:
- give your cat positive times in the carrier
- maybe get a new carrier that does not smell like fear and vet
If your cat still does not like it, do proper carrier training. Your goal is to have the cat settle down and be neither afraid nor angry.
- Put the cat in there for a short time
- Give it something it likes maybe a treat or a toy
- Let the cat out and reward it
- Repeat this with longer timespans until you reach the goal
This is important since you usually take your cat in the carrier when it is not feeling well to begin with, so it should at least be at ease with the rest of the situation.
It might also help to improve the overall carrier-vet situation if the cat associates the vet with something else than being sick and treated. Some vets offer kitten appointments where the only thing that happens is the cat going to the vet, it gets taken out of the carrier gets some pets and treats and leaves again. I think this a great way to associate the vet and therefore the carrier with positive feelings.