Yeah, forget Lavender Brown and Parvati, those FAR MORE SHINING* examples of what you've just ranted about! -_- Oh, JKR. Sometimes I really think you've got the right idea about the world, but if you're still mad at your childhood "Pansy Parkinson", all these years later, something is rotten in the state of Denmark.
OTOH, I don't think there's anything hypocritical in being thin and/or attractive and not wanting other people to measure the world by those standards. Saying it's hypocritical of JKR -- a thin, attractive blonde woman -- to rail against the cult of thin? Strikes me as a little defeatist. A little like, "you're white, therefore you can't possibly speak out for black rights" or "you're mentally whole, you can't possibly speak out for the insane". I would agree that it is possible not to understand what a particular group's situation is like -- because there are always "you have to have lived it" situations. But that doesn't mean you can't intellectually understand the problem and dislike the fact that it exists. And besides, I'd wager that JKR knows a thing or two about extra weight and being judged by it, considering she'd given birth to three children. It's so fucking easy to whine "you don't know what it's liiiiiiiike!" and dismiss people, isn't it?
Try getting a job that requires you to use your brain when you're thin and pretty. If your interviewer is male, you run the risk of feeling like a five-dollar whore for the way he looks at you. This is especially wonderful when you end up quitting because you're being sexually harrassed. If your interviewer is female, you run the risk of being judged on your looks, "oh, she's well-groomed and so pretty, she can't possibly be as smart as her CV says". I'm not even mentioning the more extreme scenarios, but I've been on the receiving end of both of these and it ain't that fucking great. A fat girl who just can't lose weight might never become a runway model, but at least she won't be as likely to be viewed as a piece of fucking meat when trying to enter a career that requires a functioning brain. It all comes down to what's valuable to you, and there are no absolutes. Frustrating, but true, so far as I've observed. Ironic, innit? The only absolute thing is that nothing is absolute. XD
But defining your life by what people think of you is unfortunate. I won't say stupid or wrong, because I can only make that decision about my own life choices, not other people's. But at the end of the day, you're the only one responsible for where you are in life. Sometimes shit happens and there are hurricanes and layoffs and evil double-crossing people, sure. But you've been given a brain so you could use it, and blaming the world or your circumstances on your life situation misses the point. When shit happens, we deal with the consequences -- how you deal is up to you. You can blame the fact that you have to deal on whatever happened, but HOW you deal with a given situation is your responsibility. We are all responsible for our own stupidity, our own failures, our resourcefullness and our successes.
When you are feeling discontent with your life, have a look in the mirror and have a good long talk with whomever you see there. Even without being psychic, I can reasonably predict that it won't be your boss who refuses to promote you, or your fifth-grade teacher who failed you in math.
ETA to clarify: * If she's going to rant against the lipstick-and-body-obsessed girls, she's better off picking Lavender or Parvati to make an example of because those are actually SHOWN to be like that in canon. Pansy isn't.