It absolutely is cruelty, many dog breeds are bred specifically for deformities that risk the health of the animals.

The actual reason for keeping track of bloodlines (aside from brainworms) is mainly to reduce inbreeding - not many dogs get to have offspring since most end up as pets or in shelters. But yea if people didn't give a shit about dog breeds in the first place and just let them mix it up, it wouldn't be as big an issue in the first place.