Cats, like all living organisms, occasionally have mutations that affect their body type. Sometimes, these cat body-type mutations are striking enough that humans select for and perpetuate them. This is not always in the best interests of the cat, as many of these mutations are harmful; some are even lethal in their homozygous form.
Some mutations in cats are fine for the cat's health.
Many of the mutations are harmful, but not all. That means some mutations are fine.