You can consider it a primary source, yes. Any novel might be a primary source. The important question to ask as an historian is, however: a primary source for what? What are you using information in the book to prove? To answer that question, it's important to understand Remarque's biography and what he was trying to accomplish. It's not a primary source on exactly how certain battles unfolded, but it might tell you something about the common soldier's attitude towards war, and anti-war sentiment in Weimar Germany.

I'll tell you, though, it's not all as anti-war as the stereotype! There are plenty of scenes where combat is more like a game or sport. It goes to show you, perhaps, how deeply combat and violence had sunken into the German spirit by the close of WW1. That's also exactly what makes it a complex view of reality - and worthy of study.

tl;dr - it is an accurate depiction, just use it with a grain of salt and an understanding of its political context.