Clowdy Jewel takes its title from the main character, Julia Cloud. It's a nickname given to her by her young niece and nephew.
From the Grosset & Dunlap jacket flap:
Julia Cloud, whose life has been given to caring for her faAmily, is left alone by the death of her invalid mother. Having always lived a dutiful, useful life, she has been invited by her married sister and brother-in-law to come to their home, virtually to become a drudge. At the moment, however, when her new slavery seems inevitable she is visited by her niece and nephew...they joyously make plans to thwart the designs of the married sister...a new and beautiful environment opens before "Cloudy Jewel".