Of course the Hemings mitochondrial DNA was passed to her kids. But so was some Jefferson maternal DNA.
The mitochondria at real issue is Jefferson’s own, from his mother, grandmother, great grandmother ad infinitum. That is what they are trying to hang Jefferson’s purported fathering of Hemings’ issue on.
But the Jefferson mitochondrial DNA would have been inherited by a lot of the Jefferson line, and that is the source of the controversy.
Jefferson mitochondrial DNA could have wound up in Sally Hemings children through possibly eight different men who carried Jefferson mitochondrial DNA and may have consorted with Sally Hemings.
If it was Thomas, it is not a big deal. He would have been engaged in a legitimate marriage if it were possible under the laws at the time.