Norman Vincent Peale wrote a book called "The Power of Positive Thinking". Johnny Mercer was a famous singer and song writer; one of his songs was called "Accentuate the Positive" The author of the sentence you quote is saying that nowadays people think that the idea that you will achieve a lot if you persist in being cheerful and optimistic (thinking positively) is a bit idealistic, unrealistic and even silly, but he himself thinks there is perhaps some truth in it. "were on to something" = had actually made a discovery, were on the right track.

What the author is saying is that focusing on good thoughts and not bad ones is something that is viewed by most people today as "old fashioned" and something that was mainly done during the 1950's, and that Norman Vincent Peale and Johnny Mercer thought like that as well, and that maybe they were right to do so.