Having seen/heard some truly awful Stevie performances, (ahem, Red Rocks, anyone?) that others have thought were so wonderful... I have to say this is NOT really that bad. I think, as someone else pointed out, we are only getting one channel of the audio, and we're just mostly getting Stevie's mic. You can't hear Chris's keyboards really at all and Lindsey's guitar solo is echoey and a bit muffled in the mix. This makes it sound like she is bellowing and drowning everyone else out but I gotta blame the recording and the mix on this one.
She does sound like she's singing it more how she does solo than how she used to do it with FM before touring solo.
But she's not so way off key as plenty of other performances.