'chunky monkey blubbernaut' is what you said, not the above. It's judgmental and quite frankly, just rude. Nothing about 'victimhood syndrome'.
You can be a proponent of body positivity and make changes to your own body. But for all you and I know, Trainor might have made changes for health reasons? Therefore, you've been judgmental in thinking she's a hypocrite just because, in your words, she 'now looks really good' after being overweight. She looks good in your mind because you clearly don't like bigger women. Otherwise I doubt you'd have called them 'chunky monkey blubbernaut'.
They're proponents of body positivity, as in feeling confident and happy in your own image. Maybe it's stress-related, or health-related that their body has changed. You don't get to decide what to call them just because you didn't like how they looked.