Definitions by The Polish Grammarian

Bumper Sticker Mentality 

An inability to examine an issue in depth. An inability to get beyond the basic thought, which sticks in their head like a mantra, closing off all other avenues of consideration.

Due to Larry's bumper sticker mentality, he was unable to understand the nuances of an issue beyond a basic premise, as in "guns don't kill people, people kill people." He lacked the ability to think beyond the bumper sticker.

Bumper Sticker Mentality by The Polish Grammarian December 11, 2015