I do not think it was fat shaming. She rather complimented Rani for being comfortable in her own skin and whatever weight she has now.
Rani is not slim. She has been looking plump and aged for last 2 or 3 years. In Durga puja events she looked older than Kajol who is actually older than her.
Yet Rani wears her weight, wrinkles, dark circles, on her sleeve. She is unabashed and unashamed of it. She does not try various ways to hide weight or use botox or fillers or sport a certain hairstyle to look young or hide weight.