Like Commodus' disrespectful behavior in the Senate chamber, Maul's posture on the throne emphasizes his illegitimacy. Maul is not Mandalorian and can only rule behind the quisling Almec. He gained his position by trickery and murder. Moreover, he has no respect for Mandalore or its people. He treats the center of Mandalorian political power not as a sacred place but rather as conquered territory. For him, Mandalore is simply a springboard for his campaign against the Council of Neutral Systems.
The scene also reinforces Maul's tenuous hold on sanity. In a recent interview on RebelForce Radio,* Maul voice actor Sam Witwer reminded viewers that, deep down, Maul is still the pathologically insane being we saw on Lotho Minor. However, throughout history, rulers have used political authority to hide their insanity. The Roman Emperors were infamous for engaging in outrageous behavior. Commodus was actually quite reasonable compared to Caligula, who appointed his horse as senator. However, like the emperors, Maul hides his insanity behind symbols of political power. Because Maul holds political power, none dare call him insane, but instead excuse his behavior as mere arrogance.
Of course, none dare challenge Maul because he controls Death Watch, the Mandalorian equivalent of the Praetorian Guard. That is, none except for Maul's former master, Darth Sidious...
* The folks at RebelForce Radio also noticed the similarity between Maul's posture and Commodus. Glad to know I'm not alone!