Montag doesn't like the TV walls even before he starts to question society, but afterwards, he becomes acutely aware that they are not offering anything except mindless, emotional noise without actual content. Because the people of the future have been conditioned to avoid all discourse, they do not understand that simple noises and people displaying emotion without reason does nothing for intellectual stimulation.
Well, wasn't there a wall between him and Mildred, when you came down to it? Literally not just one, wall but, so far, three! ... And the uncles, the aunts, the cousins, the nieces, the nephews, that lived in those walls...
"That's all very well," cried Montag, "but what are they mad about? Who are these people? Who's that man and who's that woman? Are they husband and wife, are they divorced, engaged, what? Good God, nothing's connected up."
(Bradbury, Fahrenheit 451, Google Books)
Montag correctly intuits that the "relatives" in the walls are simply making noise without reason, and trying to induce emotion in their audience for no purpose other than distraction. Now that Montag has more of an individual mind, he is able to understand that the TV offers nothing to understand. This is a deliberate tactic by the government; they can't have people exposed to contradictory ideas, which are endemic in fiction, so they focus on noise to make the audience think they have experienced something significant, when in fact they have experienced nothing at all. This is a perfect example of Shakespeare's famous quote from Macbeth: "a tale told by an idiot, full of sound and fury, signifying nothing." The "stories" on the TV walls are meaningless and full of sound and colors, but offer nothing to the viewer; this is a deliberate choice by the government to force independent thought out of the populace.
The people in the walls are Bradbury's version of futuristic reality TV. Much like our own mainstream entertainment, the gigantic monitors lull Mildred into a vacuous world of violence and sex. Montag watches Mildred watch the screen. He longs to scream out that they are not in love and that their relationship is as fake as the tripe she is addicted to watching. Montag escapes to his books but at one point challenges Mildred if she has any clue about what is happening outside her walls and asks "Is it because we're having so much fun at home we've forgotten the world?". I love the sarcasm and irony in that line! In the final chapter Montag comes out of his enlightened closet. He turns off the walls and starts spouting off poetry to Mildred and her friends.