I'm of the opinion that if you judge somebody on their physical appearance, doesn't matter if they're kermit the frog, it still makes you a shallow and unpleasant individual.
Fatness is demonized so much in the media, yet more and more people are overweight, and there's a myriad of reasons for being overweight. Over eating is just one of them, but sometimes people are so quick to judge that they don't even stop to think there could be other reasons - for example, a friend of mine has a terminal brain tumour, and the steroid treatment she has been given to try and save her life has made her weight balloon up. Now, she's technically obese - but is that her fault? No. She gets comments all the time like "stop eating you fat pig" and it's very hurtful.
That's just one individual and I know it's an unusual case, but I think if you're happy with how you look it can make your whole life more enjoyable. Being bullied for something that really has no effect on anybody but you is purely spiteful and there's no benefit to it.
As for a moral right though, I don't know what you're getting at. You have the right to call anybody a fat pig if you wish to, but I certainly think it makes you a bad person with a cruel streak. What's that old saying, "if you can't say anything nice, don't say anything at all"