Mrs. Johnson is the wife of Mr. Johnson. Although she has never been seen on Sesame Street, Mr. Johnson at one time attempted to have her painting framed. The portrait depicts a woman of advanced years, with completely gray hair and smiling through a pair of dentures (complete with gold tooth). Thus, Grover initially takes her for Mr. Johnson's great-great-grandmother, until he's corrected: "That's my wife!"
Mrs. Johnson's absence in the series could suggest that she is perhaps bed-ridden, or the couple is estranged. In the very least, they don't share a fondness for popcorn. Given the fact that Mr. Johnson's home life in general is seldom displayed, however, it could simply be that his spouse does not share his predilection for dining at Charlie's Restaurant, or otherwise placing one's fate in the hands of a cute, furry, sales attendant of questionable qualifications.