Consider the differences between Mildred and Clarisse. How is one a threat to the ideas presented in Fahrenheit 451 and the other an ideal example of a Fahrenheit 451 character?
Mildred is a vengeful character and Clarisse is compassionate. In their world, war is valued and thus, Mildred's penchant for violence makes her an ideal citizen.
Clarisse is a curious and spontaneous person who enjoys thinking and intellectual stimulation. Mildred distracts herself with substance-less technology to avoid thinking, which was the reason why books were replaced by TV.
Mildred helps burn books and Clarisse helps preserve them. Mildred is an ideal character because she follows the government's instructions and never breaks the law.
Mildred is married and Clarisse is not. Being single is looked down upon by most characters in Fahrenheit 451.
Clarisse is beautiful and Mildred, we are told, is not very good-looking. Physical appearance is valued beyond all other attributes in Fahrenheit 451.