Obesity has many causes, including pollution (take a look at the Born in Bradford study results, for example) and nutritional deficit. There are also very few known treatments that are effective and produce long-lasting results.
So what you'll end up is shaming primarily poor people with limited access to nutritious foods and / or living in poor areas with high pollution and people with mental health problems while at the same time not being able to offer them any real strategies to "not be fat".
In addition, a lot of the costly consequences that are supposedly due to "obesity" are really consequences of things like metabolic syndrome, which does play a role in obesity but thin people can be affected as well. So you wouldn't even be shaming the right people.
I'm fat and I can assure you that there is no lack of fat shaming as it is. Doesn't make a difference to my weight, though.
I struggle to see how your suggestion would accomplish anything except make the world an even worse place.
Alternative suggestion: Look into Health At Every Size (HEAS), be nicer to people, help them out, lobby your council to make it more pleasurable to walk places, find ways of encouraging more local shops to sell fresh food. And for your own sake, stop worrying about other people's weight.