It should come as no surprise to the reader that in this case, the title is a key reminder about where the meaning of the novel really lies. The Underdogs in question are Demetrio Macías, a man who has a misunderstanding with a powerful business man who hires the government to assassinate Macías (or at least harass him), Cervantes, a charming educated man, Margarito, a cruel man with bad manners, La Pintada, the powerful woman warrior, and Camila, the flirty teen who always gets what she wants by manipulation. Notice that they are all powerful people, so the meaning of the novel has to do with the ironic juxtaposition of the term "Underdogs" to refer to these powerful, but outgunned people.
The meaning of the novel is the spirit of rebellion, because those who fight the government for the right reasons do so because there is corruption at the government level—wealthy men can simply bribe the law to do what they want. That means that the central description of injustice would be the disenfranchisement of the poor. That's where the Underdogs really find their heroism, after all, or else they would never have left their homes to train in the wilderness.
So the novel really is about justice, but none of the characters ever see it. It is only in retrospect, in Azuela's steady description of them encountering various obstacles and responding their own ways, that the reader can see the picture forming. The thematic picture is clear: Utopia would be if rich people weren't allowed to get everything they wanted by bribing the government. In this case, the Mexican government is merely a mythological symbol for the true source of injustice, human greed for money.
