So in the end equating a loathsome idea or concept to a Flat Earth mentality is not the best tactic because the analogy it's built on is false (similar to the false belief about suicidal lemmings)
So what to use as an alternative? Geocentricity would do nicely. The idea that the Earth is the center of the universe was very widely believed until Copernicus proved it wrong in a monumental feat of scientific genius.... yet the false belief still persisted many decades after the data was comprehensively falsified.
But the Flat Earth shorthand remains commonplace in editorial cartooning... and has been applied to a wide range of topics. Here's a small collection (to add to previous collections)
Steve Greenberg on the recent flat tax debate
And finally a pair of Non Sequitur strips (by Wiley Miller) about political discourse that ran back-to-back (so even though the first one isn't "Flat Earth" per se, it seemed appropriate to post them together)