"And why the potato emblem for the couch lifestyle? Lumpy, heavy, and inert, the tuber lounges on its soft divan, training its many eyes on the television screen, for endless hours."

http://wordsmithradio.org/scripts/couchpotato.html

"As far as anyone can tell, the term was first used in July 1976 when Tom Iacino, one of a group of self-described "indiscriminate TV viewers," coined the word couch potato, applying it to one of his indolent Southern California chums.

Taken with the name and the concept, the group of TV addicts got off the couch to appear in the 1979 "Doo-Dah Parade," a parody of the Tournament of Roses event held in Pasadena. Assembing themselves on a float carrying TVs and "ceremonial couches," the couch potatoes lounged passively, unashamedly watching television for the duration of the parade."