She is NOT in a coma. From her official website (jonimitchell.com):
Contrary to rumors circulating on the Internet today, Joni is not in a coma. Joni is still in the hospital - but she comprehends, she's alert, and she has her full senses. A full recovery is expected. The document obtained by a certain media outlet simply gives her longtime friend Leslie Morris the authority - in the absence of 24-hour doctor care - to make care decisions for Joni once she leaves the hospital. As we all know, Joni is a strong-willed woman and is nowhere near giving up the fight.