When someone is so naive, gullible and easily manipulated that calling them a mug is just too much of an understatement. In a group of friends there is often a person who gets mugged off more than everyone else or manages to get the group into bad situations when it doesn't seem possible. They would be known as the topmug of the clique in question.
Stuart: Kerry told me his fantasy league password because he wanted me to make changes to his team while he was on holiday. I took that opportunity to fill his team with the worst players in the league.
Alex: He gave you his password!? What a topmug.......