If you ask an 8-year-old--ANY 8-year-old--if they want to be fat when they grow up, they will say no. Why? Because obesity is a disease; usually a very apparent one. It usually leaves a mark, but doesn't have to be permanent. Fat does not describe a person any better than vomitting does. Certainly, you can tell someone to look for someone with an affliction, but it is, despite all arguments, not a permanent physical feature. I know a girl who weighed about 200 lbs in 2nd grade and was down to around 110 by 8th grade. (she's maintained a healthy body since)
For someone to be fat is not a flawed role model, and Santa Clause is not a role model anyway. Firemen are role models. Actors and actresses are role models. Teachers and parents are role models. Ridiculous fairy tale characters made to help children conceptualise the selfless giving of Christmas are not role models.