Are the characters in Fahrenheit 451 stereotypes?
Sure they are! In literature, we like to call them archetypes. This means they are typical character types that we see over and over in story after story. Let me explain what I mean:
Faber is a mentor. Although a teacher or instructor may not always be able to go out and perform the duties or skills they teach, they perform this role for one who can. Faber was afraid to expose himself and die for this cause, but he was willing to coach and guide Montag through the process.
Montag is a sort of hero. Although the society's problems aren't quite over, we see Montag escape the city and the problem of the firemen for now. He gets to go and continue his journey or quest with Granger and his men. Most heroes go on quests, this is what typifies Montag in this book.
Mildred is the "dumb blonde". Of course people with blonde hair are not inherently dumb, but there is a stereotype out there which has declared this untruth. She has almost no ability to think for herself or assess right from wrong in terms of morals. She can tell what the law requires of her, but sometimes even the morally right conflicts with the legally right.
Beatty is the bad guy or villian. Often, a bad guy is a couple of steps ahead of the hero. Here, Beatty knew Montag would take a book, and that he would "get sick". Sometimes bad guys have a redeeming quality in the end of a story that turns them good. The same occurs here when after Beatty dies, Montag figures out that Beatty wanted to die.