I haven't commented on this site for a few years. In fact, one of the last times I remember commenting was in another thread that concerned mitochondrial DNA. Some of the "regulars" treated me like an idiot. Frankly, it was because they had no knowledge of some of the foundations of cell biology. I'm going to make a few points and they can either do their research or dismiss me as an idiot again.
First of all, you have to look at mitochondria as a primitive bacteria that was incorporated into what are now eukaryotic cells. We're talking about an event that occurred billions of years ago. For those who are unfamiliar with this concept, it is called Endosymbiotic Theory. Why it's important to understand this concept is that one needs to view mitochondria as symbiotic "parasites" that multiply and pass on their genetics completely independently from our nuclear DNA.
Secondly, the mitochondria (and thus the mtDNA) of a mother are passed on to the subsequent generation within the cytoplasm of the mother's egg. It is a continuous line of passage through the female.
Thirdly, the mtDNA is very stable. While there is always a possible evolutionary upside for variation in nuclear DNA, there really isn't one for mitochondria. They have a small genome, and most mutations probably won't work out. In fact, most mutations will quickly result in an individual cell's death.
Based on the nature of mtDNA inheritance and mtDNA's tendency to maintain its integrity over time, I do find it possible that Bigfoot mtDNA would be indicated as "human" when tested. We frankly have no idea how closely we are related to Bigfoot. And while I'm not going to go to any length to defend Melba Ketchum's studies, I will say that any Bigfoot breeding with a human female, even if it only happened one time, thousands of years ago, would produce a mtDNA "human" result in a sample from a descendant today.