The title of this poem is definitely relevant to the poem, although Nature is really not what the poem is about. So I would say it is somewhat relevant.
Nature is a relevant title because Nature is portrayed as the kind mother who is leading us toward death. She is reassuring us that it is not scary and is doing what is best for us. So she is a major character.
But the poem is not about nature. It is about death and our human attitudes towards death. So a title that shows that might have been more relevant.
The theme of Henry Wadsworth Longfellow's (1807-1882) sonnet "Nature" is the tenderness and gentleness of 'Nature' in guiding human souls from this world to the next.
He has synthesized and synergized the thematic metaphors in a compelling manner to effectively convey this theme. The title is very apt because it very succinctly encapsulates the theme of the poem.
The two metaphors in the octave [the first eight lines] 'a fond mother' and 'her little child' are harnessed in the chronotope of the arrival of the bed time of the child. Death in this world is compared to the bed time of the child. Just like how the child would like to continue playing forever with its toys without going to bed, adults also would like to continue to be busily engaged forever in their mundane activities without any thought of a higher reality.
But the kind and affectionate mother knows how essential sleep is for the good health of her child and very gently coaxes the child to leave its "broken playthings" behind and puts the child to sleep. Similarly, death very gently leads us away from all our earthly attractions and distractions and leads us into the mysterious but higher "unknown."
Longfellow's conception of death leading adults to a higher and mysterious reality is platonic and agnostic. It is not a Christian view of life after death in which sinners will go to hell and the righteous to heaven. Longfellow's views on the 'after life' in this poem are non-judgmental and apply to all humanity.