I'd say don't be daft, but this was really believe in Victorian times as they had some strange ideas about genetics. No, it doesn't ruin the sire or dam if they produce a litter of crossbreeds. The pups aren't pedigree, obviously, but the parents can be mated again to a dog of the same breed and the pedigree line goes on uniterrupted with KC registered purebred puppies even though their half siblings were mutts.

I suppose you're lucky they are called boxers rather than brabadogs or bullbanters.

Oh no! That's what I call them too!
I have to be very careful not to say it to their owners though as I don't think they would appreciate this name.
As an asthmatic who has to have a non-shedding breed I have seen so mnay with Lab type coats and only 1 with a Poodle coat so it wouldn't be a good choice of dog for someone like me.