Yeah, it means "stop being lazy". "off your duffs" means 'get up, stop sitting on your arse'. A 'couch potato' is a lazy person.

"cough potato" is a term that refers to someone who is lazy. This is usually applied to someone who is just sitting and watching TV without doing anything productive (hence, they are a couch potato). The statement basically means to get up and do something and be productive rather than sitting and wasting your time doing nothing.