A couch potato is someone who spends most of their time watching television and does not exercise or have any interesting hobbies (Collins Dictionary).
Would it be correct to use the phrase to refer to a person who spends most of their time at home (lying on a couch) overusing social media and/or playing video games (not exercising or having interesting hobbies)?
I don't think so.
The expression "couch potato" was coined quite a few years ago to describe a person who had a certain behavior pattern that was commonly talked about at that time: namely sitting on the couch for too long watching television, specifically television.
I'm sure you are right that the objectionable aspect of this behavior is passivity and lack of physical exercise (maybe mental exercise too), but the expression is really about television. I don't believe it has been updated in common use to include smart phones.
It isn't hard to understand what you mean if you mention the term "couch potato" when you are talking about smart phone addicts, but you should make it clear by indicating explicitly that you mean video games or social media, etc.
examples:
I was becoming such an i-phone couch potato that I felt I had to join a gym.
That guy is so boring; he's a real video game couch potato.
Yes, that would be a good use of the term. The key thing is lack of physical activity-- passivity.
