Wharton compares the "darkness of death" to "a black whirlwind on which they were both spinning like leaves." What does this metaphor suggest about the protagonist's mental state?

I inferred you are referring to a story written by Wharton.

Answer:

She's mentally unconscious.

Explanation:

Note we were told a few lines before the comparison that the woman was "in a state of nervous exhaustion", and she fell "fitfully in and out of a dreamlike state".

This statement indicates that due to the protagonist (the woman) anxiety about her husband's death she became nervous and fell asleep; becoming mentally unconscious.