I agree with her that people shouldn't be body-shaming, especially when it's women criticizing other women. I find that disgusting and I can't understand it tbh.
But it's hard to take her seriously about this when she based her own career on looks and was --as I imagine-- flattered (like anyone would) about people complementing her on her looks and appearance. She never talked about those kind of issues in the past because she wasn't concerned (contrary to her ex-bandmate Nicola Roberts). You can't have it both ways. You have to expect criticism, especially if you present yourself on the first day of the X factor auditions (where all the medias attention is going to be focused on you) in an outfit who accentuates that weightloss. She's the one who keeps talking about it and people who wants criticize won't stop doing it anyway.
Having said that, I don't give any excuses to those who have body-shamed her and I can understand how hurtful it must be. You can't called someone "bag of bones" and think it's acceptable. But she's not the only one called things like that and she won't be the last, unfortunately (especially with social medias who give a platform to everyone, including hateful people).