Not casting any aspersions on any fellow ‘Naut but in the UK a mug is a term used to describe someone gullible, especially in reference to buying stuff. Sometimes called a Mug Punter. Can’t explain the image on the mug tho
Also if someone has been ripped off you might say they’ve been mugged off. Obv there’s a connection here to being mugged, ie relieved of your possessions