Technically, it's not miming. She uses the same technology that the A-list divas use, can't remember the name of it though. Basically, whenever she sings off-note or deep or high, the system merges those off vocals with a pre-recorded live-track that compensates and "fills" any missing audio.
Because this is done digitally, and requires its own separate sound mixer who's on the ball at all times, it's very expensive. So most artists don't bother.