It should be noted that this in fact is not just his personal ideology, but a stance that is widespread in Japan - and which is often used to deflect the Japanese WW2 guilt. Instead of saying Imperial Japan was awful, this gets generalized as "war is awful", which even allows the Japanese to depict themselves as co-victims, where the US nuclear attacks get more attention than what the IJA did in China.
You will find this in a lot of other anime, too, where you have a very generalized, very naive pacifism that ultimately doesn't say much, because that war as such is a bad thing, that's something most people can agree on anyway, even those who think their war os justified after all.
Now, I don't think that's deliberate on Miyazaki's part. I don't think he sets out to relativize Hapanese war crimes ir anything of the sort. It probably isn't even conscious. But on the societal level, yeah, immature "all war is bad" pacifism is a way Japanese society coped with the WW2 guilt.