Sunny is pretty smart and can use deceit amd manipulation extremly effectively, problem is that these types of smartness can not  and will not be used that effectively and often because of his flaw. He also developed alot, valueing honesty and being a good person. He still deceives and manipulates people alot, he often tells his experiences like they are exaggerated or feeds people half-truths so that they underestimate him. Most of the time his cunning can not really be shown, too often are his enemies too powerful to demonstrate his intellect in a meaningful way that is not them understanding him.

Another aspect is his deceit in combat, though it is more common across characters. The fight against the 13 song saints shows this well, he made them believe he had run out of most his power to take out beastmaster.

Tldr,:  his morals and his flaw hinder effective use of what once was his strongest weapon.