Although the novel's themes center upon Pip, Miss Havisham does embody several of those themes. For example, Dickens discusses the issues of isolation and alienation. While Pip is isolated because his social class, Miss Havisham has chosen to isolate herself from family and former friends and willifully alienates anyone who tries to become close to her. She serves as an example to Pip of what a person can become when he/she has social rank coupled with loneliness.
Miss Havisham also contributes to the social class strife in the novel. Her former fiancee--a gentleman (Compeyson) receives a lesser sentence than does Magwitch (Pip's convict benefactor). Because of this discrepancy based on social class, the novel's action is set in motion when Magwitch first meets Pip. Miss Havisham plays a more direct role in the social class strife by constantly making Pip believe that he is not good or high enough for Estella.
Because so many of Dickens' subplots in this novel are interwoven, one can connect Miss Havisham to almost every theme. In regards to her believability as a character, I think that Dickens intends for her to be a caricature, and exaggeration. While it is true that many people have been jilted in love and remain bitter for long periods of time, the idea of someone adopting a child and raising her to break men's hearts or of someone remaining in her wedding dress for years on end is implausible--it certainly makes for an interesting storyline though!