There are two kinds of U.S. ambassadors:
1) Career foreign service people
2) Friends of the presidential administration at the time
Examples of the latter aren't hard to find. Off the top of my head I'm familiar with William Timken, a US businessman who was appointed Ambassador to Germany by George W. Bush because he was a huge supporter [0,1]. Colloquially we could say he got the job because "he was a friend of Bush."
I remember being surprised to learn that US Ambassadors were (maybe still are) individually responsible for paying for a good part of their formal duties as Ambassadors(eg throwing parties, travel, etc). So historically, having wealthy people in the role was a requisite. I gleaned this from reading "In the Garden of Beasts" by Erik Larson [2].
The bits I have cobbled together suggest there is an interesting history surrounding the practice of selecting and acting as a US Ambassador.
[0] https://2001-2009.state.gov/outofdate/bios/t/53349.htm
[1] https://www.spiegel.de/international/spiegel/us-german-relat...
[2] https://www.goodreads.com/book/show/9938498-in-the-garden-of...