I would say, to a modest degree.
I mean, I think you know perfectly well that policy at NATO is really set in capitals. Foreign and defence ministers are pretty heavily involved in shaping policy for NATO, both on specific operations and in general terms.
Ambassadors can amplify that. They can take initiative to some degree. Ambassadors of the UN are somewhat more empowered, as a typical rule. Certainly ambassadors can make a difference, but I would be modest about that. It's really capitals that drive policy in most instances.