Food for thought, but needs a mature reader. Violence and difficult ethics/morality.
Katniss has made it through two Hunger Games, only to discover that with Peeta in the hands of the evil Capitol, and herself in the hands of the rebels, she's still very much a player. Katniss has been asked to be the "Mockingjay"- the symbol of the rebels that will inspire the impoverished districts to cast off the slavery of the Capitol. Mockingjay leads the reader through the civil war of Panem through Katniss' eyes, showing a very dark side of the war and of the minds of its victims.
The morality of Mockingjay can be summed up in one word - Complex. Katniss recognizes that oftentimes people on the "good" side, including herself, do wrong things for the wrong reasons, and struggles with this. She also struggles with the concept of "The Ends Justifies the Means" that she's been living by her whole life as the consequences of everyone else in Panem living by these rules come to life. Because the book is all told from Katniss' view as she struggles to keep a hold on her sanity, much of the morality is left to the reader to make sense of.
There is no God or gods in Panem, unless you count the god of money and power that dominates many character's motivations.
More than the other books. Even though most of the deaths aren't children (though children do die), the body count of Mockingjay is very high. Characters die in sometimes terrible ways. Characters are tortured. Katniss kills a possibly innocent person. Mockingjay is a depiction of war with the "cool battle" effect stripped away.
Drug and Alcohol Content
Even though Haymitch is sober for most of this installment, he still does reach for the bottle sometimes. Katniss and another character struggle with a painkiller similar to morphine, called morphling.
Katniss kisses Gale and Peeta. Another character confesses selling his body in exchange for the lives of his family and friends. There are some places that could be seen as a veiled reference to something sexual, but not explicit.
Crude or Profane Language or Content
Katniss looses her temper many times and characters are said to swear, but what they say is not recorded.
Mockingjay left me with mixed feelings, so much so that I had to read it again before I felt clear-minded enough to write a review. Many people describe the feeling as "being emotionally gutted". Mockingjay left me with a lot to think about, and those thoughts will vary from person to person.
If you're reading this review because you've already read "The Hunger Games" and "Catching Fire" and want to know if it's worth going on, it is. Many of Katniss' moral loose ends are brought into a new light as she searches her conscience.
If you're reading this before starting "The Hunger Games" wondering how dark the series gets, it's very, very dark. I would only recommend it for mature teens or adults. It gives a lot of food for thought, but you have to be able to sort through the (intentionally) flawed morals it leaves for the reader to sort out.
If you're reading this having picked up Mockingjay without reading the first two books and want to know if it's any good, please see the reviews for the first books in the series.