Interpret the following sentence about Mary Maloney: “There was a slow smiling air about her, and about everything she did.”

That particular line of text is meant to further highlight Mary Maloney's pregnancy.  It is often said about women who are pregnant that they have a particular "glow" about them.  The glow has been attributed to skin clarity, to a brightness of the eyes, and to a general overall happy demeanor.  Whatever the cause of the pregnant "glow," Dahl is calling attention to it with the line in your question.  Everywhere that Mary goes there seems to be a light happiness about her and everything that she does.    It even affects the very air that surrounds her.  A reader doesn't know that Mary is pregnant, when the reader reads the line in your question, but Mary is six months pregnant.  That detail is given in the next line along with several other pregnancy "glow" descriptions.  

Her skin -for this was her sixth month with child-had acquired a wonderful translucent quality, the mouth was soft, and the eyes, with their new placid look, seemed larger darker than before.