In Season 1 Episode 4 of Adam Ruins Everything Adam Conover says this about John Augustus Larson commenting on the polygraph machine,
[Larson] would go on to regret inventing it. Larson became so horrified by law enforcement's unscientific use of his device he would go on to call the machine a "Frakenstein's Monster." - Adam Conover, Season 1 Episode 4
So did Larson regret inventing the machine, and did he call it a Frankenstein's Monster.
It seems his source is, Truth in the Machine, Three Berkeley men converged to create the lie detector.
It wasn’t just his protégé he renounced; Larson regretted ever inventing the apparatus. Shortly before his death of a heart attack in 1965, Larson wrote, “Beyond my expectation, thru uncontrollable factors, this scientific investigation became for practical purposes a Frankenstein’s monster, which I have spent over 40 years in combating.”
Also reproduce in Ken Alder's, The Lie Detectors: The History of an American Obsession
Beyond my expectations [...] thru uncontrollable factors, this scientific investigation became for practical purposes a Frankenstein's monster, which I have spent over 40 years in combating.
New York times repeats a shorter version also without providing the original source but in the context of a book review of the The Lie Detectors (cited above),
But toward the end of his life John Larson, inventor of the machine, despaired. He called his work “a Frankenstein’s monster, which I have spent over 40 years in combating.”
I can't find the original source of this claim, or where Larson wrote it.