This quote points to the themes of destruction and a lost generation. In chapter 5, Muller questions his comrades about their plans for "after" the war. Though this is a seemingly innocent (and somewhat commonplace) conversation starter, it is actually received with mixed emotions. Unfortunately, because of the death and destruction they've not only witnessed, but experienced, none of these boys will ever be the same again. Though they talk of things like women and drinking, the truth is, most of them cannot imagine life after the war.
When he says, "The war has ruined us for everything," Kropp is talking about the comparison of life before the war to life after. Before the war, these boys were students who may have had short or even long term goals that likely included typical things like careers and families. The sheer trauma of what they've experienced as a result of fighting on the front lines however, has put many of the "childhood" goals and dreams in perspective. In the face of that amount of death and destruction, in the face of raw fear for their lives on a semi-regular basis, it is no wonder none can imagine a regular life again.
This quote, though spoken by a fictional character, embodies the simple but profound emotions that such serious stress and trauma cause. Kropp sums up the whole of the war in one word: ruin.