I understand that Kubrick wanted to play with the dehumanizing effects of the military/war, and this is arguably one of the reasons there is not much characterization, but I don't think that jives, or is really that fair to the viewer, when there actually is a de facto main character in Joker. It's like the movie wants to have it both ways--the main character is this mostly faceless group of soldiers in green, and the main character is Joker. In trying to split it, it fails in both ways. You never feel that the ensemble is an ensemble, a group together; and you never feel like Joker is really the main character. Like you said, that narration by Joker; most people would probably not even realize that Joker was the main character if not for that.
I'm all for ambiguity, and playing with the way stories are told in a visual medium, but I think Kubrick's ideas were just muddled, or not fully developed here. It's not a bad movie, but it's not nearly as striking, or mentally stimulating, as any of his other pictures. It also doesn't look or sound so hot. This is the movie he came closest to flubbing.