If I lived in the Arctic, I think I'd want quite a few layers of fat to keep me away/make me look menacing to polar bears. I reckon people should abolish these perceptions or what is healthy and what is not based on looking at someone. My father weighs over 16 stone (not all muscle), yet his blood pressure, heart rate and general fitness are great. Being round is not the only indicator of "poor" health.
Santa Claus is supposed to be a fun, jolly fatherly figure who gives presents and toys and loves his milk and cookies. I don't think anyone is looking at Santa and wanting to share his physical appearance. Even young children look at him as a father or uncle type figure, not something to grow up into.
Santa Claus is not a role model, he's supposed to be a fun fatherly figure. It's okay to portray him as fat as that adds to the idea of his warmth and the impossibility of sliding down the chimney. Additionally, there's enough negative associations with different body types already, there's no reason to add Santa into that mix.
I think it is healthy if Santa if fat. Santa has always been heavy set and I do not see a problem with it. Santa is one of the most recognizable fictional characters and is loved by millions of people. Nobody has ever complained about him being fat before so I don't see why it would be a problem now.
Although we often attempt to change the depiction of various characters in media in an attempt to raise awareness of issues like obesity, this is not the appropriate course of action in the case of Santa. While many children find Santa Claus an amusing or comforting character, I do not think there is any serious trend of children attempting to copy him.