Is it ethical to "adopt" a cat? My cat lives a good life, she is old and healthy and well feed, never having to worry about anything. But it is ethical to own another living being, even if you offer it the best possible life it can have in return? I really don't know.
But if you do adopt a cat, I think you need to do everything that ensures the cat has a healthy life, like getting shots and check ups etc. Neutering the cat is also an important part of keeping it healthy. After all, it greatly reduces the risk of either gender getting cancer in their reproductive organs and a lot of other issues.