It is interesting how often ISFP and INFJ comes up for Plath. Both share Ni and Se where you take in the sensations of the external world and distill them into an inner symbolic vision.
Another thing should be said of great artistic expression. It typically contains within it enough layers to hold up under different interpretations of it. It could well be that her poetry can be viewed from multiple angles and maintain a sense of coherency. It is akin to viewing a cube from multiple angles where one person sees a square, another a diamond shape, and still another a hexagonal, each being entirely correct. That quality is inherent in great creative achievements.