From Muppet Wiki

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 grey hair and smiling through a pair of dentures. Thus, Grover initially takes her for Mr. Johnson's great-great-grandmother, until he's corrected: "She'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 ones's fate in the hands of a cute, furry, sales attendant of questionable qualifications.