Could this be The Marching Morons by Cyril M. Kornbluth?
First published in Galaxy magazine, April 1951, it was included in The Science Fiction Hall of Fame, Volume Two. In this story, the "time traveler" was a cold sleeper from the 1980's.
In 1988, real estate agent and con artist John Barlow is placed in
suspended animation after a freak accident. He is revived in the
distant future, in a confusing world filled with hypersexualized
advertisements, vapid entertainment, and people who exhibit erratic,
nonsensical behavior. Shortly after being revived, Barlow is
introduced to two men who tell him that the current state of society
is the fault of the "morons," the world's vast population of
unintelligent people, who greatly outnumber the much smaller
population of intelligent people.
These men explain to Barlow that the most urgent crisis of their time
is the population problem ("Poprob"). Historically, people of higher
intelligence often chose to have few children (or no children at all)
for pragmatic reasons, while people of lower intelligence, compelled
by their sex drives, had larger families and reproduced in greater
numbers. By the far future era in which Barlow has awakened, this
unbalanced trend has been carried to its logical extreme, with a total
world population of five billion morons (with an average IQ of 45)
living under the supervision of three million members of an elite,
intellectual upper-class who secretly govern world affairs.
Project Gutenberg has a copy here.
Together with Frederik Pohl, Kornbluth later used this idea in their 1954 novel Search the Sky. In this story, the intelligent elite hid in plain sight as janitors and bathroom attendants.
Ross fished absently in his pocket. “The thing that bothers me, Doc,”
he said, “is that I know there are intelligent people somewhere
around. I even know what they’re doing, I bet. They’re doing exactly
what I tried to do: acted as stupid as anybody else, or stupider. I’d
make a guess,” he said, warming up, “that if we could just make a
statistical analysis of the whole planet and find the absolute
stupidest-seeming people of the lot, we’d——”
He ran out of breath all at once. His eyes bulged.
He looked at the men’s-room attendant, and at the ten-cent piece in
his own hand.
“You!” he breathed.
The attendant’s face suddenly seemed to come to life. In a voice that
was abruptly richer and deeper than before, the man said: “Yes. You
had to find us yourself, you know.”
Project Gutenberg has a copy here.
