What might be seen as a pessimistic theme in Fahrenheit 451?
The culture of Fahrenheit 451 is a very pessimistic view of the future. The government has banned books and taken full control of television; people have lost all their ambition and drive to succeed and instead sit around watching meaningless TV shows all day. These shows don't even have plots, just conversations about nothing; even Seinfeld, famously "about nothing," had plots throughout, but the entertainment of the future -- which people are satisfied with, and have no ambition to change or create something better -- is literally about nothing.
"...the man says, 'What do you think of this whole idea, Helen?' And he looks at me sitting here centre stage, see? And I say... 'I think that's fine!' And then they go on with the play until he says, 'Do you agree to that, Helen!' and I say, 'I sure do!' Isn't that fun, Guy?"
(Bradbury, Fahrenheit 451, Google Books)
Montag asks what the plot is; Mildred responds: "There are these people named Bob and Ruth and Helen." That is the entire plot. In this world, no one strives to create something new; no one tries to challenge the culture or its preconceived notions; no one remembers life before the banning of books; no one cares about politics or the state of the world around them. Instead, people are content to sit and be fed worthless emotional cues by the government, which actively prevents people from expanding their personal knowledge, and kills those it deems dangerous to society.