There's many sayings about diplomacy, though I understand the reality is much more mundane.
One that comes to mind is "a diplomat is a person who can tell you to go to hell in such a way that you actually look forward to the trip" — like all good quotes, attributed to a wide variety of famous people.
Competent governments send arse kissers to those who need pampering, and send blunt to those who need to see bluntness. But (in a competent government) these things are uncorrelated with the actual negotiation position — "speak softly and carry a big stick" etc.
Trump being bellicose to everyone at the same time is a sign of his own incompetence.