No.This is one of the more enduring and farcical JFK theories. Researchers have checked out the sewers of Dealey Plaza and determined that a gunman would not have been able to fire over the windshield of the approaching presidential limousine. As far I can tell no living serious JFK researcher believes in this hoax.
The gunman in the sewer is like the “Secret Service man did it” and “George H.W. Bush did it” and “the Federal Reserve did it.” It’s fiction.