I mean, yeah, the dialogue doesn’t present it as a failure, but it doesn’t take much to come to that conclusion, why did she pick politics when she would’ve been much more helpful as a Jedi?
Well the idea that a Jedi is much more helpful is not really a point of fact. Seems more like Leia sought out Luke when she realized the Resistance wouldn’t be enough to stop the First Order alone. And even if the Jedi were around, they alone wouldn’t be enough either, they needed the Resistance too, and that came from Leia.
In TFA both the Resistance and First Order believe that Luke’s return alone would change the tide of the battle, even though TLJ does bring this into question, the fact is Leia could’ve done more against the Fist Order as a Jedi than as a senator. The New Republic failed to take action and the Resistance was a last ditch effort to combat this emerging threat.
What good did her diplomacy do to the New Republic that was obliterated two movies ago?
Again this is a decision we already know that Leia has made going on two movies at this point. Unless the issue is with the specific phrasing of “best serve the galaxy,” I don’t see how this line changes anything.
Before this movie we aren’t given any indication Leia was ever anything close to a Jedi, she seemed to be in the same place we left her in RotJ, naturally in tune with the Force without any formal training, her space walk in TLJ is closer to Luke pulling the lightsaber on the Wampa cave, which he did before any real Jedi training. It would’ve been fine if they stuck to that but in TROS she suddenly had all this Jedi training that she chose to never use, so there needs to be a reason for that. The film decided the reason was that if she picked up a lightsaber her son would die, which is ridiculous, so I think there needs to be something equivalent to replace that.