Back before the tracklisting was revealed and everyone was hating on Poor Mabel, I wrote what I thought about this song - turns out I wasn't wrong
but anyway this was my take on it.
Stevie had to have written this in 1984-1985, while she was dating Joe Walsh. Mabel Normand was a silent film star, the first comedienne, woman director, and launched Charlie Chaplin's film career. In the early 1920's, she was possibly heavily addicted to cocaine (Hmm) and found much unhappiness in her life. Her story really is very sad, ending with her untimely death from tuberculosis in 1930, mostly brought on from weakness she sustained from her fast-living lifestyle. I think Stevie saw this, and recognized it was a reflection of her own life.
The actual verses talk about losing her love. She says, "and I call you Beloved Exile." Beloved Exile is a book about King Arthur, or more specifically, the death of King Arthur, leaving behind Queen Guinevere to live alone, in her misery. I remember in an interview she gave in the 90's I believe, saying that both her and Joe would have died if they had not broken up, because of their ridiculous lifestyles. "So continue on your destructive road."
I think she compares herself to "Poor Mabel Normand" and Guinevere, and Joe to King Arthur; two completely unrelated people, but whose lives are parallel. As strange as it is, I completely understand every single word in this song, and I have since the first time I listened to it. It is one of the most important songs to me, and it always will be.