The issue is that we have so, so many stray cats. So, so many cats left in shelters. We do not need people to breed more cats just so people can have their very expensive and very specifically bred designer furbaby. We need people to take care of the ones that already exist and are already struggling.
Creating designer pets is immoral. Creating a side channel market for abandoned designer pets is immoral because it increases demand for designer pets (people can abandon their pets knowing they will be cared for).