Not necessarily.
If the immune system is fine in an unbred state ... and if it's fine even in a bred "once or twice" state ... then where is the problem?
If a problem only ensues after a bitch gets bred multiple times in succession ... never being allowed to fully recover from the last time ... and after a period of several years finally starts getting rundown and moth-eaten ... then the problem isn't really with the bitch, the problem is in her being over-bred. Essentially she is being rundown by her owner.
Some dogs simply get terrible mange, and I agree it is undesirable. However, such dogs usually get it early on, and they struggle with it forever.
But IMO this is a whole other topic than a bitch who is flawless and perfect for 6 straight years of her life, but only AFTER being bred over-and-over-and-over again starts to fall apart.
Jack