Can someone who is not a career Foreign Service officer be a good choice as Ambassador to a UN body or other non-country entity?

The key qualities you look for in an ambassador - ability to negotiate with multiple parties with divergent interests, a drive to implement policies, even temperament in stressful situations - cross-cut occupations. Being a career Foreign Service Officer certainly has its advantages, particularly in having a leg-up in navigating the bureaucracy and politics of the State Department, but good diplomats aren't exclusive to the world of foreign service officers. Fundraisers, for example, probably have very keen networking abilities and the capacity to operate in multiple spheres of influence. That's a huge skill to have in general, and in diplomatic organizations in particular, and part of why they get brought to the President's attention for nomination (although the money they rake in doesn't hurt). But the real quality that a president wants in his ambassador corps that is satisfied by fundraisers and political operatives in a way that edges out career servants is loyalty. Now, that doesn't mean Foreign Service Officers are somehow inherently disloyal or wouldn't perform as required by the president - not at all. But that's out of their commitment to the job at hand. However, top fundraisers and key political operatives and backers have shown a personal loyalty to the president that he can rely on when push comes to shove. Again, is it fair to assume that a Foreign Service Officer wouldn't be reliable if called upon? Of course not. But for the president it's an issue of known versus unknown variables. When presented with candidates who are all capable diplomats, the president will of course trend towards those with whom he has an existing relationship and rapport. It's not a commentary on the capabilities of Foreign Service Officers, it's politics. Thank you for the A2A.

Absolutely. One thing that should be remembered is that many of these positions usually require a level of technical expertise that a career foreign service officer may not have. For example, working in the International Civil Aviation Organisation, someone with a background running an airport is exactly the type of person you want there. This sort of person, assuming they are skilled enough to get to that position in the first place, has the necessary technical expertise to be able to discuss issues such as airplane standards, while at the same time had enough political nous to be an effective representative. I would point out that some of the positions you mention in the question details are filled with career civil servants, those who have extensive government experience, usually in the area of the organisation they are heading up. In my limited experience, these people can be quite effective if they understand their role as the conduit between the government of the day and the organisation. However, they can get into trouble if they hold a bias towards one or the other. This usually takes the form of being too beholden to outdated (or unelected) political ideologies. But this is rare.

It should not matter if chosen well. Even Shirley Temple was made a ambassador.

I am a senior foreign service officer with twenty-five years experience.  I'd state unequivocally that a political ambassador can be outstanding -- indeed, I've worked with many of them -- but they must be selected on the basis of their professional expertise, not on the amount of money they gave to a political campaign.  Wealthy political donors sometimes don't make good ambassadors; they don't understand their role.

NO It is a tradition for Republican Presidents to select a person on record as against the UN's existence as US Ambassador to the UN. It is an integral part of populist American ideology that the amateur, someone like Jimmy Stewart or Forest Gump, can do as good a job as a pro. Both Republicans and Democrats select rich party loyalists who have only a vague notion of the country they are sent to. Both the British during Empire and the Americans of the middle 20th C were loud in their ignorance and contempt for the local food, language, and religion. They deliberately mis-pronounced the local language. Tea party Republicans have revive this tradition of being proud Ugly Americans. The HBO series  on John Adams shows how even sincere intelligent educated people with the best intentions can be a disaster in the field of diplomacy. There is too much damage that can be done by amateurs. The damage done by George Bush and his appointments will last 20 years.