When Beatty states this right at the beginning of the third section of the novel, they are standing in front of Montag's house. A fire alarm has been called in on his own house. Montag is stunned, and at this point, he turns his head and looks at Clarisse's house, and how dark and abandoned it looked. He is probably comparing their house to how his will become: empty, devoid of life, a shell of what was before. He realizes that "they" (probably the government) has probably taken the entire family away, or worse, in order to silence them. Beatty notices Montag's glance, and when he states "You weren't fooled by that little idiot's routine, were you?" he is referring to Clarisse. Beatty is pretty bitter towards Clarisse. He claims that her "type" likes to go on and on about life, about "flowers, butterflies, leaves, sunsets," and suck you in with how different and unique they are. They bewitch you with talk about life, happiness and beauty, and it throws your entire world upside down. Beatty remarks, very cynically that Clarisse is
"One of those damn do-gooders with their shocked, holier-than-thou silences, their one talent making others feel guilty."
Beatty's anger and acidity seems a bit out of place here; he rants and raves against Clarisse and her "type" of person, people out there in the world that make others unhappy simply through their example of happiness. He attributes a malice and manipulation that is unfair to Clarisse; where Clarisse was innocent and sincere, Beatty reads her as conniving and scheming. Since Clarisse was the catalyst to Montag's transformation, she obviously did have quite an impact on him, and maybe Beatty has seen that before, and is angry that so many people are prompted to rebellion because of "nonconformists" like Clarisse. Beatty feels that her innocence and beauty were all part of an elaborate routine that he has seen before, and that is used to undermine their society.
I hope that those thoughts helped a bit; good luck!