I think it’s fair to say that diplomats appear to be appointed under a two-faced system.

On the one side you have some diplomats who really are quite capable career foreign policy wonks, appointed in a manner which appears to be meritocratic.

On the other side you have folks appointed, like you mention, as a kind of patronage.

Traditionally, it has been that the softer counterparties (Friendly countries, European allies, small island nations, etc) are staffed with patrons while the more difficult or geopolitically sensitive relationships are manned by professionals, but this is certainly not always true, and one can find many counterexamples.