I think perhaps Katie Clark found a good balance between seemingly too obviously impaired and hardly seeming impaired at all.
Re whether Margaret had overreacted: Of course, it's not just Margaret. It's also Roy. I think we are meant to believe that Clara's impairment is significant and real.
Have you seen the film version of the novel? It seems to me that Yvette Mimieux as Clara is rather obviously impaired, and it is odd that the Nacarelli family never seem to notice anything off about her.