Realism, Dystopian Literature, Tragedy
We can definitely state with no ambiguity that All Quiet on the Western Front is notcomedy, Children's Literature, or a Fairy Tale. It is not postmodern, experimental, or a romance. But it could be just about anything else. Some things to consider:
One could argue that it is a coming-of-age story, but, if so, then it's equivalent to a story about a baby being suckled by porcupines. Paul comes of age but then, poof, he dies. Usually coming-of-age stories result in the main character learning a lesson, which he then takes to heart and passes along. This is like a coming-of-death story, or something analogous.
And it certainly could be labeled a Tragedy. There isn't anything not tragic in the book. A daisy doesn't even grow in the end.
But at a higher level, All Quiet on the Western Front might best be labeled Realism or Dystopian Literature – the battlegrounds are as close to those of 1984's mind-numbers as anything else. This is the complete opposite of utopia. What makes this story so powerful is that the story was, for all intents and purposes, REAL. It documented with great clarity the details of WWI as the world had never yet seen or heard them.