The practice of giving ambassadorships purely in exchange for (large) campaign contributions was seen as somewhat problematic even in the US, which passed a law in 1980 attempting to mitigate this. But that law was apparently very easy to circumvent, with the practice of large donors becoming ambassadors exacerbated (again) under G. W. Bush and Trump. (Do note that this practice of an apparent quid-pro-quo with large campaign contributions is different from merely political ambassadorship appointments, which have a broader tradition in the US, even though that tradition in itself is less common in other countries.)
Some but not all Democratic candidates for the 2020 election have pledged to substantially curtail the practice.
More interestingly perhaps, from the paper that Relaxed linked, it's also been a bit of a tradition to appoint ambassadors to "friendly countries" like Canada, Japan, and some Western European countries, but also to Mexico, in part based on campaign contributions. The paper also has some stats on campaign contributions from future ambassadors.
While noteworthy that 73 percent of political nominees donated, it is perhaps more surprising that 27 percent did not. The common assumption seems to be that those who become ambassadors without first entering the Foreign Service must have paid their way into office. The surprise largely dissipates, however, upon closer scrutiny. A clear majority of the 27 percent are individuals with high-level experience in the executive branch or Congress—such as Deputy Chief of Staff at the White House, Chairman of the Joint Chiefs of Staff, and members of the House or Senate—all of whom appear capable of using their influence or personal connections to the president, rather than financial support, to secure a nomination. [...]
The picture for career nominees is much different. From 1980 to 2019, only 5 percent personally contributed, and their contributions averaged only thirty-three dollars. [...]
Another way to think about the role of money is to examine whether campaign contributions affect bilateral relationships to different degrees. Figure 4 addresses this issue by depicting, for each state, the percentage of the nominees who made financial contributions to the nominating president, with values ranging from 0 percent (white) to 100 percent (black). The data establish that donors were most common among nominees to politically stable and economically developed countries, particularly in Western Europe.
This doesn't, of course, answer whether this is ethical or a good idea, other than to justify it with tradition.
There are also some stats over time on contributions by would-be ambassadors:
This pattern (of contributions) is a bit different than merely looking at the percentage of political appointees. G. W. Bush and Trump are clearly outliers here with much larger campaign contributions (from would-be ambassadors) than the rest of the presidents since Reagan.
Also Sondland is not actually the largest such example from the Trump era:
United Nations ambassador Kelly Craft and her husband contributed over $2 million to Trump’s election campaign and inauguration. She also gave generously to over half the Republican senators on the Foreign Relations Committee that had to approve her nomination.
Some but not all Democrats do seem to see as unethical the appointment of large donors:
Democratic presidential candidate Sen. Elizabeth Warren has said she will appoint no big donors as ambassadors — period. But when I have contacted the campaigns of every other person seeking the nomination to ask if they would make a similar pledge, I have been met with silence. That is because, in Washington, money does the talking.
And according to a 2017 study cited by the Washington Post, the US is indeed an outlier with this practice of politically-appointed ambassadors, compared to other countries:
A 2017 study by researchers at the University of Texas at Austin said the United States was an “extreme outlier among foreign services in the number of political appointees as ambassadors, even in key posts.”
The researchers noted that in the other countries they studied (Brazil, Britain, China, France, Germany, India, Russia and Turkey), ambassadorial posts were almost entirely reserved for career diplomats.
And the newspaper also quotes some quid-pro-quo examples of appointments in exchange for donations:
Herbert Kalmbach, an attorney for President Richard M. Nixon, once testified under oath that he had spoken to a wealthy political donor about an ambassadorship. “Well, you know, I am interested in Europe, I think, and isn’t $250,000 an awful lot of money for Costa Rica?” Kalmbach recalled the donor, Ruth Farkas, complaining about one potential destination.
Farkas eventually donated $300,000 to Nixon’s reelection campaign. She was ambassador to Luxembourg from 1974 to 1976.
That appointment caused its own quid pro quo scandal. The Foreign Service Act of 1980 attempted to address it, stating a chief of mission should “possess clearly demonstrated competence to perform the duties of a chief of mission,” including language ability and knowledge of history and culture. “Contributions to political campaigns should not be a factor in the appointment of an individual as a chief of mission,” the act states.
Despite the Foreign Service Act, political appointees, many of them big-dollar donors, have continued to be placed in embassies since 1980.
So yeah, there was an apparent attempt to mitigate the practice in the US by at least requiring some qualifications from ambassadors with that 1980 law, but apparently it was easily circumvented thereafter.
The fact that the Senate confirms ambassadors has not proven much of check in practice:
In theory, an ambassador requires Senate confirmation. This can weed out some picks. Though the Senate has not voted down a White House ambassadorial pick in decades, just appearing before the Senate can sometimes cause weak candidates to crumble: Hotel magnate George Tsunis withdrew from the nomination to become ambassador to Norway in 2014 after questioning revealed his lack of basic knowledge about Norway.
But, in general, the Senate does little to hold things up. The ever-ballooning cost of U.S. electoral campaigns, which surged to $6.5 billion in 2016, according to OpenSecrets, may add to the problem. Many wealthy presidential donors who harbor ambassadorial ambitions also donate to members of the Senate Foreign Relations Committee.
I'm not sure there are stats on the latter kind of donations (i.e. from would-be ambassadors to Senators).