It's used in informal conversation in BrE, and is fairly common. "I've been such a couch potato this week, I've been watching Netflix all day!" 😊

I agree with Trudi. It's also used in American English, it's a common expression and is used quite often in the US.

I don't know how often people use this sland expression in their every day life. As I know this expression is about a lazy person who spends a lot of time laying on the couch and watching TV

it's a metaphor for someone who is lazy and spends all their time on the couch watching TV or playing videogames.

I live in America, and English is my native tongue. Here in the US, a "couch potato" is a term used to refer to a person who spends much or all his/ her time on the couch. It can mean the person is lazy, but in my understanding, its not a particularly insulting term, but I think that would also depend on who you are referring to , and how that person might take it. An example of a non derogatory comment could be, "I worked so hard yesterday, that today I'm just going to be a couch potato".