how is Juliet a character who matures as the book progresses

I am not sure you can use her growth as an example of a "growing up" (bildungsroman) story because truly the story is over so quickly that even though she deals with grown up issues (marriage, family feuds, first love, death,disappointment), she is truly not mature.  We would only have a real bildungsroman if we saw her after a time of being married to Romeo and a time of learning how to deal with her parents and his family as they resolved their differences.