most dog breeds have changed for the worse, not just pugs. a lot of this is because of people breeding for show dogs instead of working dogs
mixed-breed dogs, breeds with less mutations (flat faces, dwarfism, excessively long faces, etc), and dogs from popular breeds but from working lines have much less issues.
something i found a while ago about dog breeders that kind of disgusts me is that dogs can be in almost any color, but those that don't have "breed standard" colors are considered inferior, even if they are perfectly healthy. people will insist that a dog has to be mixed breed, or that a dog is from a puppy mill or something if the dog is a non-standard color
a white lab
blue german shepherd
any dog can be born with a "chimera" mutation that gives them random black spots. it's completely normal but they are also considered inferior