Fats, apart from discipline, also lack fashion sense. Another reason is these Tumblrfats rarely get out of the house, and when they do, they quickly rush back to eat (I read somewhere that some reeing Tumblrfats feel bad eating in public because of fat oppression or something). So when they do manage to wobble outside they feel like they need to document it for the world to see, even in such basic outfits that no normal human would consider "fashion".