In Avengers: Endgame, Nebula says that Thanos was many things but not a liar, but why would a trillion-murdering maniac refuse to bend the truth?
In Avengers: Endgame, in the wake of his destruction of the Infinity Stones, Nebula claims Thanos is not a liar, but why would someone willing to murder trillions be so reluctant to betray something as simple as the truth? The answer lies not in his moral integrity, but in his arrogant sense of honor and self-importance. The revelation was less an affirmation that Thanos was in any way good, but rather speaks to the real driver of his mass murder of half the universe.
The moment in question comes when the Avengers trace Thanos to his so-called garden sanctuary, where he has retired in the wake of the snap. His act of destroying the Stones alerts the surviving heroes to his location, and their discovery of the fate of the Stones prompts the angry accusation that the Mad Titan is lying about what he did. That prompts Nebula to say "My father is many things. A liar is not one of them." Given their closeness, there is no reason to question Nebula's assessment, and Thanos' reaction - of surprised thanks - further suggests that she spoke the truth.
Looking throughout his appearances in the MCU, Thanos does not appear to lie: he uses the Reality Stone to deceive the Guardians of the Galaxy, but that isn't quite the same and it seems he is committed at the very least to his own believed truth. And picking away at Thanos' surface confirms why it would be the case: his commitment to the truth is an arrogant belief in himself. He sees lying and deceit as dirty tricks, not befitting an honorable warrior, which he paints himself to be and he fundamentally believes himself to be above such conduct.
Thanos' reaction to Nebula confirms what he thinks of liars in a round-about way because he confesses his treatment of her was perhaps too harsh in response to her confirming his integrity. Considering his open derision of her and the emotional and physical abuse he put her through, dehumanizing her, the fact that he would reconsider at all is a major testament to how he values truth. It is beyond a compliment, it's worth a fundamental change of treatment, which is particularly surprising given Thanos' unflappable convictions. After all, this is the same character willing to kill his beloved daughter to achieve his ultimate end, who weakens himself fatally after achieving it, who is dangerously blinkered in his murderous agenda. That he would express regret is telling.
But not only that, Thanos confirms what he thinks of those who would turn to deceit and betrayal - which includes lying - in his treatment of Loki at the start of Infinity War. The only times Thanos kills anyone up to that point is in the service of his agenda: he kills at random to balance the universe, so as not to appear a hypocrite and to prove fundamentally that he was right all along (despite his world's rejection of his plan). Even when faced with Hulk and Iron Man, he fails to kill, resorting to murder with Heimdall because he is standing in the way of his plan. Loki, on the other hand, is punished for being as close to a walking personification of lying as the MCU can muster. Thanos even seems to take pleasure in it, because he's dispensing with a creature who deserves the worst because of his lack of integrity.
In Infinity War and Endgame, Thanos' whole plan relies on the fact that Thanos believes himself to be a paragon of integrity and conviction, in his own eyes. The greatest slight against him, in that same assessment, was Titan's rejection of him as their savior. Thanos sees himself as perfect, rather than an abomination, and the very idea that he would resort to something as underhand and primitive as lying would be a fundamental betrayal of that self-image.
