The Question and Answer section for The Importance of Being Earnest is a great resource to ask questions, find answers, and discuss the novel.
I have never considered Cecily silly..... she's simply a girl who acts like any other. None-the-less, her writing and avoidance of schoolbooks allow us to infer that she's far more interested in fiction and fantasy than serious thought....
She thinks Uncle Jack does not joke or crack a smile. She thinks he is so serious that he might be unwell.
Dear Uncle Jack is so very serious! Sometimes he is so serious that I think he cannot be quite well.