Why do you think that Bradbury would introduce Clarisse before montage's wife Mildred?

I'm not exactly sure but I think it is because Bradbury wants to show the clear contrast between the 2 characters. While Clarisse appears cheerful, his wife has just attempted suicide. Clarisse is outcasted by the others because she is "anti-social", but in truth she is not. Guy notices that he is unhappy with his life (the question that Clarisse asks), and that his wife isn't either. No one is happy in reality; they all live in fear of he government and do not talk to others.

Another view is;

Clarisse is introduced first in Fahrenheit 451 because Clarisse is more important in the development of Montag's character in the novel than Mildred. Mildred plays a very small role in Guy's realization of his dis satisfaction with his life. He does not really love Mildred anymore, but he shows a clear attachment to his 17 year old neighbor Clarisse. Of course literature, especially those expressing such clear political and social views as Fahrenheit 451, is open to many different interpretations. I believe this is what makes author's writing such as Ray Bradbury's so powerful. So you decide for yourself.