If the girl reflects what Montag feels, what must Montag be feeling? Was he happy?
I don't think Clarisse is so much a reflection of Montag feels as a foil. She shows him the magic that reality can contain, as opposed to the empty images offered by society at large. When Montag first meets Clarisse, he believes he is happy. It is only after she shows him true happiness that he begins to question his entire existence. This is the inciting force for Montag embarking on his journey.
In the beginning of the book, Montag's happiness is compared to the beauty of fire and its ability to cleanse absolutely. Montag doesn't really know anything; he is pure, like fire, in that way. But after he meets Clarisse, he realizes that happiness is not as easy as catchphrases or as simple as watching the parlor walls. True happiness also requires knowledge of true pain and suffering, because people emerge from these hardships and become truly grateful. If you think about Mildred, she should be perfectly happy: she has everything she thinks she needs. And yet, we all know that Mildred is one of the saddest characters in the novels.