Montag’s memory of Clarisse is compared to his view of the empty wall. She was an empty, characterless person, made so by a basic lack of mental development resulting from a tyrannical government and empty way of life. In other words, the lack of literature and free thought has created in Clarisse an empty and, one might even say, soulless person.
By extended comparison, this is also a comment upon the way in which society in general operates. The invasive and controlling government that dictates all input to society has created empty and soulless individuals, whose very individually is as lost as that of an empty wall.
The significance of this comparison relates to the general theme of the book; literature and free thinking are a requirement to maintain the mental health of individuals and of collective society.