Well, if the dead civilians come from immoral leaders ordering schools bombed .... yes, that's something to oppose.
But if it comes as part of the fight, as part of the unavoidable cost to defeating evil, then of course it is no reason to oppose war.
So, what we discover is, this "reason" has no substance. It depends on another claim. And it adds no useful information: we already know to oppose wars by murderous folk, and support wars by the righteous.
So, opposing the war based on too much collateral damage, is just judging the US to be morally bad, combined with hiding one's meaning behind a smokescreen.