I think (it's hard to tell without actually seeing the scene you're referring to) Phoebe is just being silly in that there's an exclamation mark in front of a person's name to indicate some kind of seriousness. It didn't have to be Freud, it could've been Mark or Watson and she could have said it's not just Mark it's Mark!!!!!!!! Chandler just wants to say that Rachel must never have been to Joey's show before because she is very excited and calm (implying that Joey's show is always horrible).

Chandler is being disrespectful. He might be serious or joking. When you are waiting to see one of Joey's plays, you should feel worried. Either Chandler thinks that Joey's plays are bad, or else he thinks they are very depressing. Rachel is feeling a sense of cheerful excitement and anticipation. Chandler thinks that if Rachel knew what Joey's plays are like, she would not be feeling that way. The title of the play is apparently "Freud!" with an exclamation point after it. Exclamation points indicate excitement, surprise, or strong emotion. We don't know why we are supposed to be excited or surprised by the name of the dour psychoanalyst, Sigmund Freud. We know that it is not a straight biographical play, or the title would just be "Freud" or "Sigmund Freud" without an exclamation point. Putting an exclamation point here seems incongruous. Phoebe is worried about what to expect. Is it going to be an ironic joke (like the play title, "Who's Afraid of Virginia Woolf?") If so, she doesn't think an ironic joke about Freud is going to be entertaining. Is it going to be some kind of comedy--like the musical comedies "Fiorello!" or "Oliver!" She doesn't think a comedy about Freud would be a good idea. Using an exclamation point is sometimes a cheap or amateurish way to add excitement when the writer couldn't write something exciting. Perhaps the title is "Freud!" with an exclamation point because Joey couldn't think of a good title. In some sort of way, she thinks the exclamation point is lazy or cheap and doesn't bode well for the play.