How does the title fit into the overall themes of Les Misérables?

The title refers to the basic level of underclass and
disenfranchised that exists in France.  Hugo's title takes the French word and apply it
to the level of society that lacks power and a voice.  It is Hugo's hope that he is able
to bring a voice to the voiceless, power to the powerless, and the acknowledgement to
those who are invisible to the upper echelons of French society and to French
government.  The title helps to bring to light the suffering of a group of people. 
Their predicaments in living in the shadows of French life and how it feels to be
marginalized is the primary purpose of the work.  In the process, it is important to
understand that Hugo is deliberate in speaking the words of an entire group of people. 
The violation of human rights to this group of people, the "miserables," is in fact
"miserable."  It is something that need to be brought to the attention of the reader and
in the process it is one that is explicated thoroughly in the course of the narrative. 
The title is reflective of how those who are deemed as "miserable" have a narrative and
possess a story to tell.  In making this his title and focus, Hugo feels comfortable
enough to do so.