Can I ask my aunt to pay for the vet bills for the kitten she gave us?
Why did she give you a cat?
She's selfish. She only cares about herself.
You accepted the cat when your aunt gave it to you, so it is wrong for you to now go back and ask her to help pay for the bills. Take responsibility for your choices and pay for your cat's care.
I didn't want the cat in the first place.
If you didn't want the cat you shouldn't have taken it. Your aunt shouldn't be paying for the vet bills for a pet that you willingly accepted. If you can't take care of the cat you can find it another home.