It can be used to describe a disgusting face and/or a fat person.
Alex: "I´m getting wasted and i´ll try to get laid with some schweinekopf downtown. Wanna join?"
Pedro: "No dude, thats disgusting - why would you do that?"
Alex: "Because schweinekopfs are easy to score!"
Alex: "OMG .. guys look at that fat chick!! She´s the most beautiful schweinekopf i´ve ever seen! i think i have a boner!"