They make a stupid face so that they can use it as an excuse when people say that they look bad, instead of just being ugly in the first place. Can you imagine how they would feel, if the fats tried to look as nice as possible, but still have people say that they look like shit? So this is just a defence mechanism to protect their fee fees. The middle finger is to show how "quirky" and "sassy" they are, which is another defence mechanism to convince themselves that they don't have shitty personalities.