A careful reading of the short story "The Ones Who Walk Away from Omelas" by Ursula K. Le Guin can help you answer this question about equating happiness with "simple folk." The narrator describes a beautiful city that appears to be idyllic in every way. It is in...
the midst of a joyous celebration. People of all ages have come out into the streets to dance, sing, play music, and parade through the streets. The citizens of this society are so mature and intelligent that there is little need for rules, laws, and overbearing structures of authority. There is enough technology to make life easy, but not enough to be unsightly or cause pollution. In short, the city seems perfect, and all its inhabitants seem to be happy.
However, the supposed joy of the city's population is not simple at all. The narrator explains that in the basement of one of the buildings, a child is imprisoned and tortured. This child lives in fear, filth, malnutrition, and neglect. Citizens in Omelas know that it is there and that the happiness and prosperity of the city is somehow dependent on keeping it there. In other words, the citizens of Omelas accept the trade-off of a poor, innocent child suffering endlessly in exchange for their own well-being. The people who cannot accept another's misery as the price of their own happiness are the ones who leave the city, the ones who walk away from Omelas. They go away and they never come back. They realize that true happiness cannot be bought by intentionally degrading someone else.
We see, then, that the supposed happiness of the people of Omelas is a complex cover-up. They are always aware that the happiness they seem to enjoy comes about only through an innocent child's pain. How could anyone who is sane be sincerely happy in a situation like that?
The Merriam-Webster Dictionary defines something that is "simple" as "not hard to understand or do; having few parts: not complex or fancy; not special or unusual." The Urban Dictionary clarifies that a "simple person" is "someone who is uncomplicated. They're grateful for the little things in life. They don't try to impress, they're humble." This is the opposite of the people the narrator describes as the citizens of Omelas, who share a terrible secret of torture and deprivation as the price of their happiness. We equate happiness with simple folk because simple people are honest, straightforward, kind, and merciful. They would not allow someone else to be relentlessly tortured so that they could live in supposed joy.