I would expect the mutation rates of mitochondrial and nuclear DNA to be about the same, unless the mitochondria have additional repair proteins not involved with nuclear DNA, or the shorter length of the mitochondrial chromosome affects the rates, or some other process I'm not familiar with. I think maybe parent meant to refer to diversity, not mutation. With nuclear DNA, the offspring chromosomes are made up from a combination of the parents' DNA. There's a lot of swapping and mixing and matching that takes place. This adds diversity to the population.