One of the themes in "Janus" is that of aesthetics. Most successful authors have a personal theory of aesthetics, a philosophical theory that describes the nature of beauty. As Cambridge Dictionary Online puts it: "the formal study of art, especially in relation to the idea of beauty." Dictionary.com offers a more elaborate and revealing definition of aesthetics:
the branch of philosophy dealing with such notions as the beautiful, the ugly, the sublime, the comic, etc., as applicable to the fine arts, with a view to establishing the meaning and validity of critical judgments concerning works of art, and the principles underlying or justifying such judgments.
the study of the mind and emotions in relation to the sense of beauty.
Ann Beattie is a minimalist, and since minimalism in literature opposes the traditional mimetic view of literature--that literature conveys divine truths that human hearts desire but can't find on their own without the inspired help of poets/writers (Aristotle, Sydney)--Beattie was heavily criticized by those looking unfavorably upon the new style of minimalism. Therefore, one of the themes that pro-minimalist critics find in "Janus" is a thematic statement of Beattie's theory of aesthetics.
This view equates the bowl with Beattie's minimalist style: it is perfect though not obtrusive and, yes, perhaps overshadowed by other more splendid bowls, yet it has dazzling qualities that catch the eye and hold the attention and beg to be studied, like the surprising flecks of sparkling color in the glaze of the bowl. This theme shows that Beattie's theory of aesthetics is the epitome of the "less is more" theory.