It's been discussed here before, but apparently the fatties make weird faces so they can blame that on their horrible looks and detract from the fat.  I think it's mostly lying to themselves rather than other people...cause the other people just see a fatty making a stupid face.  Usually they seem to open the mouth like exaggerated yelling/laughing. In this case, it's extra stupid because it makes its face look fatter by increasing the cheek to mouth-width ratio.