Clarisse isn't crazy in Fahrenheit 451. She just seems like it to the people in the novel, so that's how they label her.
She pays attention to nature, takes walks for fun, likes to talk and have conversations, isn't afraid to talk about love and other feelings, and belongs to a family that sits in their backyard and talks.
Members of her society, on the other hand, sit for hours in front of mindless television (hmm, sounds familiar), don't have real conversations, don't consider nature, don't walk for pleasure, and don't think for themselves.
Clarisse is unusual for her society. Thus, they label her crazy.
There are a couple of ways that you can look at this.
On the one hand, Clarisse explains it by saying that she is 17. Her uncle tells her that everyone that age is crazy.
But in terms of the society, people think she is crazy because she is so different from everyone else. The thing with Clarisse is that she likes to think. She likes to be alone and to take things slowly. She likes to get to know people in a meaningful way.
Overall, she is not like the other people in her society who rush around doing all sorts of "fun" things rather than thinking and feeling.
Because she is kind of special, and not like her sorroundings, unlike others she thinks well before she does anything.