Or perhaps you think it is acceptable to point out people's physical features and make fun of them 'because it's true
People here mock Carrie's hair, her clothes, her dry skin, her makeup, her smile. Why do you draw the line at weight? She is overweight, it's not a judgement it's a fact, like her hair are frizzy, her granma clothes or her dry skin. The comment sent to her was rude and unnecessary, but as long as the tattler isn't the instagram poster, why are you policing them? If she was underweight no doubt people would tell her she is a disgusting anorexic and that she should eat a burger and most people would be okay with that.
The quid pro quo for posting online (and getting free candles) is people having opinion about you, your appearance and your life, sometimes nice comments (the overwhelming majority in Carrie's case) and sometimes not so nice.
Imo Carrie is jumping on that comment because it plays into her 'people saying negative things about me or my life are haters/trolls including all the comments about Joel being a weirdo'. The comment was probably a random troll but now she uses to discredit all the genuine worries people have about her relationship.