Where and how did Diana learn how to be a diplomat? Everything we've been shown of Themyscira during her life there shows the island as inhabited by a single polity, and one ruled by an immortal (mostly absolute?) monarch at that. You have the interpersonal issues inherent to, well, people, but none of the kind of conflict between social or political groups deriving from competing ideologies or sets of interests for which workable compromises must be found.
Even worse, it's a warrior culture, so physical conflict, although within heavily regulated boundaries, is absolutely accepted, and perhaps even encouraged.
So why should their Princess — who rather than a misfit was highly respected and extremely well-trained by the standards of her society — be any good as a diplomat? I don't question her desire for peace at both social and personal levels, nor her reluctance to fight given alternatives, but effective diplomacy, even when the goal is peace, isn't just about wanting it, it's a skill. It's a form of politics, and Themyscira looks like an awful place to learn politics; too immortal to have to deal with heir hairiness, too small for fights over centralization and delegation of power. And their economy, to whatever degree we know it, looks like "Ancient Greece with magic replacing slaves" (I think), which doesn't even give you space to the Pandora's Box of an urban commercial class or, later, a proletariat.
Options: (1) Diana spent quite a bit of time in Patriarch's World learning the nuts and bolts of diplomacy (the Trevor timeline forbids it, but Diana could be old enough for her to have shadowed Talleyrand). (2) The Amazons have a secret school of diplomacy they trained her in before she left the island (but this only punts the problem; do they travel to keep in practice, and/or learn new tricks? that goes against the grain of Amazonian isolationism). (3) Diana is bad at it and knows it (but I've never read her say anything along those lines). (4) Diana is bad at it, doesn't know it, and neither Clark nor Bruce have been able or willing to tell her. Clark probably wishes he could approach the issues as she does, but can't due to his own standing in the world (basically, the Pope can host meetings, but he can't draft treaties). And I bet Bruce sees her directness as, most of the time, useful in the kind of negotiations he makes after scaring the crap out of people in their "highly secure" offices. In the sense of "you might possibly outflank Superman given his cultural, psychological, and geopolitical constraints, but then your options are *me* and the relentlessly idealist Superman-level highly-trained warrior with diplomatic immunity."