I do think that the script doesn't make him look like a buffoon, but it looks to me like the actor was directed to play him as hesitantly as possible, to make Kirk look better. I think the problem is in the direction. Those same lines could have been delivered decisively but they weren't. The impression that a viewer was intended to get, the way that scene was acted and directed, is that Harriman was completely incompetent.
No, I think the intended impression was that he was inexperienced. Those are profoundly different things.