I think that might be a little unfair. When she stole the document from her husband, she was simply thinking about getting her own letter back in exchange. She perhaps talked herself into believing that the document wasn't that important, even though logically she must have known it was.

But when she comes to see Holmes, she does ask him: "Is my husband's political career likely to suffer through this incident?" and "One more question, Mr. Holmes. From an expression which my husband dropped in the first shock of this disaster I understood that terrible public consequences might arise from the loss of this document... Of what nature are they?" She now accepts what an awful thing she's done.