Mitochondria contains its own DNA and codes for the tRNA and rRNA. Hence the mutations are found in the RNA transcribed from the mito DNA.

If the mutation occurred in the tRNA or rRNA, then only that one RNA molecule would be affected, with probably no defect to the organism. If the mutation is in the DNA which codes for the RNA, then it is replicated and passed on to mitochndria resulting from fission, daughter cells, and (if in the germ line cells) future offspring. But when we specify where a mutation in the DNA is, it is common to specify the protein (or RNA) coded for by that DNA, as "a mutation in hemoglobin".