It was ranked #16 in 1966-1967, #18 in 1967-1968, and #27 in 1968-1969.
1966-1967 - https://en.wikipedia.org/wiki/Top-rated_United_States_televi...
1967-1968 - https://en.wikipedia.org/wiki/Top-rated_United_States_televi...
1968-1969 - https://en.wikipedia.org/wiki/Top-rated_United_States_televi... says The Smothers Brothers Comedy Hour was ranked 27.
There were NO "cult following" shows in prime time in the top 30. Maybe on late nights.
I mean, https://www.ranker.com/list/best-1960s-cult-tv-series/ranker... lists Smothers Brothers as a cult show.
To be fair, Smothers Brothers is not listed at https://en.wikipedia.org/wiki/List_of_cult_television_shows while Deadwood is.
On the other hand, that Wikipedia list includes Miami Vice (ranked #9 in 1985-1986 https://en.wikipedia.org/wiki/Top-rated_United_States_televi... ), The Golden Girls ("ranked among the Nielsen ratings' top ten for six of its seven seasons" https://en.wikipedia.org/wiki/The_Golden_Girls ), and X-Files (ranked #11 in 1997-1998, https://en.wikipedia.org/wiki/Top-rated_United_States_televi... ).
And X-Files regularly shows up on lists of cult shows, like TV Guide's at https://www.tvguide.com/news/reacher-season-2-release-date-c... .