Let us be very careful with how we use the word "hypodiegetic" in this instance. Diegesis is a term used to refer to narratology, and in particular indicates the way in which the plot is set at a different level from the narration itself. Hypodiegesis is used to refer to an embedded tale-within-a-tale that is said to constitute a lower level of narration.
Of course, the tale itself does not strictly offer us with a framing narrative as the point of view is far more complex than this. We start of with Utterson's attempts to discover the truth that lies at the heart of the mystery concerning the relationship between Mr. Hyde and Dr. Jekyll, and then we move to Richard Enfield's account of his own attempts before finally we hear the truth of the matter from Dr. Jekyll himself. Both write about the case reporting what they have discovered. There is a sense in which, therefore, we can talk of the narration being diegetic, as Utterson talks about what he did, recounting his earlier actions, but I am not convinced that we can use the word hypodiegetic to refer to the narrative of this compelling story.