To approach disparagement, we must first back up to a fundamental aspect of human nature. We humans are social animals. Social groups naturally sort themselves into social or dominance hierarchies. Social psychologists can show that if you put two people in a room and give them a task, they will quickly and sometimes unconsciously sort themselves into a leader and a follower. The hierarchy of deference can take various forms, and the dynamics within the group can be complex, so social groups do not clearly fall into linear pecking orders based on physical intimidation (indeed, even in the original 1922 Schjelderup-Ebbes report that coined the term "pecking order," the order within a large group was often not strictly linear). Nevertheless, there is always within a small group of humans a social structure where some in the group defer to others.
I believe in our culture we tend to downplay this aspect of human nature because we value ideals of equality, liberty, and freedom. The observation that, in a group situation, one person will always defer to another goes against our ideals in the abstract. The democratic idea that the leader is chosen by the followers and is therefore the first among equals allows us to reconcile our nature with our ideals.
Because higher social status has its benefits, the role of higher status is sometimes contested. Roger Gould (Collision of Wills, Chicago, 2003) observed that a great deal of interpersonal violence occurs in contests over things that do not appear to have a great deal of economic value. The object of the conflict is however a symbol of social status, so the fight was really over the relative positions of the participants in the social hierarchy. In order for a group to act together with trust and cohesion, the threat of open internal conflict needs to be controlled.
Play occurs in a safe environment. Humor usually signals deference and a non-threatening situation (which is why the villain's evil laugh is especially unnerving). Self-deprecating humor signals to the group that the speaker does not insist on a threateningly high status. When we laugh at the pompous individual, we are lowering the person's status within the group. The word pompous itself implies that the person is trying to claim a social status higher than the person deserves. When we laugh with our group, we develop a sense of bonding and cohesion. Studies have found that within a group, even the target of laughter often feels more group cohesion as a result of being laughed at.
Within this context of group status hierarchies, one can see that the disparagement aspect of humor serves as a regulator of group dynamics. While some theorists of humor characterize it as aggressive, one can see it as a mechanism for mediating social tension in most cases without resorting to violence or group division. Which explains why most spontaneous laughter in small groups is not associated with mirth; it is a signal of deference. Nevertheless, the goal-directed, dominant individual often discourages humor and values seriousness. There are not a lot of knee-slappers in the Christian Bible, with its uber-dominant God the Father.
Michael Mulkay, in his excellent book On humour: its nature and its place in modern society (Blackwell, 1988), expanded the issue of the incongruity in humor into a broader social and philosophical theory. He argued that in normal, serious situations, one interpretation of reality is considered correct, and others are looked upon by the group as defective and possibly worthy of suppression. Humor can open up a space for conflicting interpretations to coexist without causing a rift in the group.
Mulkay did not add to his theory the concept of social dominance within the group. The group's dominant interpretation of reality is usually put forward and enforced by high status individuals, so competing interpretations of reality can be threatening to the status structure. Humor can allow alternative interpretations to be introduced without challenging the top of the dominance hierarchy, though humor can indeed be used for a challenge. (In Nazi Germany, it was illegal to name your dog "Adolf," for example.)