If a Doctor can determine the cause of death for the death cert, there is no need for an autopsy.
An autopsy is very invasive, and should only be done if necessary.
Many bereaved people no not want their loved ones remains to be put through the procedure.
Do you have any reason to believe that your mothers death was not from heart failure or that there were any circumstances that should have been investigated.
I am sorry for your loss. Even if it is a few years ago, maybe you should talk to the Doctor about this, if it is preventing you from getting over the pain of your mothers death.