Epigenetics is real, but not to this extent, i don't think.
Most breeders and genetics labs take what is called a, "mother clone." This mother clone turns into the mother plant that they use to breed, after letting the plant it was clipped from finish flowering, and the breeder sampled it to see if that's what he/she was looking for.
Now, if you clone from that mother and turn it into a new mother plant, say, 10 times, then yes. It will have become weaker, but i'm not sure if it effects yield. I know making a cultivar too inbred will effect yield and possibly make the next generation of seeds sterile, but I'm not too sure about epigenetics doing that. It's still fairly new to the cannabis world and people are researching it to a better extent.