In a sense, it is appropriate to call Clarisse antisocial. She does not want to do all the things that are normal for that society. Therefore, she is antisocial. But this is ironic because she is really more social (in our way of thinking) than any of the people who actually fit in to her society.
If you think about what makes her antisocial, it is mainly things that we would pretty much value in our society. She cares about other people. She is interested in what they really are like. She wants to know, for example, if Montag is in love and she finds it sad when he is not. You can contrast this with Millie and her friends who do not seem to care for one another on any sort of personal level.
So Clarisse is called antisocial because she wants to actually connect with people on a personal level. To our way of thinking, this is ironic because in our society being social is about connecting with people.