A person who could be very well-informed and high-functioning, possibly even quite intelligent in certain areas, but is an idiot nevertheless. Usually a
smooth-brained person has never thoroughly examined their beliefs, is generally susceptible to ideology, and has enjoyed a relatively comfortable life free of struggle or adversity.
Smooth-brained can also describe an article, film, or any other piece of content that presents a view of the world that betrays the author's complete alienation from any of the material conditions that define most peoples' lives.
Can also be used as a noun, for example:
smooth-brain.