I also was curious about the significance beyond this:
Why would he leave that up after all these years? In fact, why even stay in the same house?
Why was the mattress on the floor? And the smiley face was really low on the wall, maybe this was his daughter's room?
And why sleep here? How could he sleep there?
Thoughts that came to mind:
Maybe because he feels guilty, bringing them into Red John's radar with his public comments...
Maybe because he vowed to never forget; this is a horrible, daily reminder...
Maybe because he vowed to keep everything the same until their killer was caught...
Maybe because, in a very creepy way, he feels closer to them there, where they lived their last moments.
I was watching it and just horrified when I saw the smiley face, and more horrified yet when he lay down underneath it and went to sleep!