Sylvia Plath is renown in the literary world for her poetry, short stories, and novels. Her works are confessionalist in nature (meaning that her work dealt with personal subjects (which tended to be very taboo for the period)). Questions regarding the most significant issue or idea within a text will vary. Different readers will connect to the meaning of a novel in different ways. Ask ten people about what they think is the most relevant idea behind Sylvia Plath's The Bell Jar and chances are one will get ten different answers.
For me, the text offers a very real and frightening look at both self-identity and insecurity. Both centering around the self, Esther's path leads readers through her personal insecurities regarding socio-economic status, female identity, mental illness, and illusions/reality. Esther's hatred for her mother is quite poignant given its ability to propel her forward in life. I appreciate how honest Plath was in creating Esther's character.
Therefore, the most significant issue/ idea that is brought up in the novel is the self. Esther's "self" faces so many different things that to ignore her inner being would be a disgrace to the novel, the message, and the author (personal belief of course).