literary techniques - What is it called when a character is portrayed opposite the stereotype for comedic effect?

There is the word counterstereotype.

The Wikipedia has this page for the word, where it is defined as:

A counter-stereotype, reverse
stereotype, or anti-stereotype is the
reverse of a stereotype. Although
counter-stereotypes arise in
opposition to stereotypes, they may
eventually become stereotypes
themselves if they are too popular.

EDIT: note that counterstereotypes are not necessarily used for humor. They can be used, for example, as a protest against the stereotypes. The Wikipedia page that is linked above has some interesting examples.