
A lot depends on where on the tail it is, and how bad it is now. I had a puppy a couple of years ago with a tail kink that was noticeable, but it wasn't huge. I kept the pup as I only had two bitches to choose from and I had promised the other to a friend, for showing. I kept thinking that when she grew and the fur was longer, it wouldn't be something anyone would notice. I'd had one similar pup many years earlier and unless you felt the tail, you'd never know once he was adult. This one however, hers became a LOT more noticeable as she grew. Doesn't help that she also carries her tail over her back (a fault in the breed) as with the strong kink, her tail makes her look like a scorpion. But it is in the third quarter of the tail so it's no problem for her - except obviously she cannot be shown, nor will I breed from her.
However, back when I was breeding pedigree cats, I once had a kitten with a strong kink near the base of the tail. My vet told me that I would need to tell the buyers of that kitten to be very careful, because if for instance they (or a vet) would hold the tail upright in order to take the temperature or empty anal glands etc, the location of it meant that if it was done too roughly, it could potentially damage the spine.
So in a nutshell, if you're wanting a pet, and the kink is in the middle or towards the tip of the tail, then it is only cosmectic and nothing to worry about.