If you are overweight and unhealthy and you don't care, while straining healthcare resources, then yes, you could be considered immoral. If you are overweight, care and want to change, whether you are capable or not, then I don't think it's immoral.
I don't think abusing people by calling them 'fatties', or berating them is any help. They've heard it all and if it bothered them, they would have worked on it already. All you're doing is bullying and encouraging that behaviour if in front of children.
