Okay. I mean, it starts out, and she's this "wholesome" woman. A sex worker. It's a suiting combination because Johns aren't inclined to think they're doing anything wrong. Rather, they'd prefer believing that their sexual pursuits are healthy. A "wholesome" chick makes that delusion more attainable. Thus her original popularity, and her (profession) downfall upon (personal) descent.
Now, like. Of course there's symbolism, duh. We have a typical woman, she could be representative of many people, etc. Her body starts to change, sometimes it's good sometimes it's bad. Her reception by society changes. And again, sometimes it's good, sometimes it's bad.
There is, however, a constant throughout all of these physical and perceptual changes - her lack of control; from the word go, she is at the mercy of wheels whose motion is seemingly predetermined - her experience in life pretty much trifling. Just a ripple in the sea of fate.
So back to this symbolism thing. When this chick's a pig, it just means that she's an individual without autonomy. She is unable to control the course of her own life, and as a result, her life is insignificant (like a pig's).
There's also another issue at play here: Feminism. Her job as a sex worker, and her self image (pig) are symbolic of women's place in society, as well as how they feel about being there. It's an extreme example, but an exaggeration by no means.
The book even starts out with an apologetic tone; nothing says Subservient Woman more than "please forgive me."
When she's a pig, this is not her "accepting her body for what it is!" The freedom she experiences as a pig represents the freedom you'll experience in ignoring your own Will in submission to society. Of course it's easier to float with the current, but true humanity comes from forging your own path.
Just an aside: I recently found out (first hand...) that "Boutiques" of this nature do exist. They do employ women whose personal image is shit because of their work. And they do distribute drugs to clients, all under the guise of legitimate business. It's not too cool.