She's a celebrity selling a product. Whether she's happy or sad shes promoting a lifestyle. She can be obese and happy or obese and sad either way she selling that shit as part of her brand and there's people that look up to her for it.
Many whom take on the thinking that being obese is just okay which it isn't.
She also consistently post videos of herself working out, talks about eating healthy and things she does to keep her shit in check...so going by your logic she actually isn't promoting an unhealthy lifestyle because what she's selling is somebody who despite being bigger still manages to have physical capabilities associated with people who are smaller and assumed to be in better health. Seems like the issue most have is simply her being happy with herself regardless of size because as you said "being fat and seemingly happy" isn't something people like to see.