This is a good question. This is a great work and as with all great works, the answers are not easy. In light of this, Cormac McCarthy's Road is both a book about despair and hope. There are many elements of despair. The very fact that it is a post-apocalyptic world shows despair. There is much death and dying. Moreover, even those who have survived care little about civilization. For example, there are roving bands of cannibalistic tribes. This is despair with a capital D.
The book is also about hope, because the main characters are filled with hope for a better world. The father and son show hope in the face of despair. There is also incredible love, sacrifice, and care. In fact, the father's love for his son, even though he is dying is amazing and deeply moving. Moreover, they are hopeful for a new civilization to emerge.
In light of this, I would say that if I had to choose, it is a book about hope in the final analysis. It is the theme of the hope of a few to make the world a new place again.
Is there any evidence or examples of hope from the mother commiting suicide?