There is an interesting look at humour in the theology of Karl Barth at

http://theologytoday.ptsem.edu/oct1986/v43-3-editorial.htm

Note particularly section III (page 311-12).

It makes three good points;

1. That humour should be self-directed because "Humor is the opposite of all self-admiration and self-praise" (CD III/4, p. 665)

2. That humour is "laughter amid tears" because it "presupposes rather than excludes the knowledge of suffering" (Ethics, 511)

3. That humour is a "humor is grounded in the grace, faithfulness, and promise of God. Humor is part of the freedom which is ours to exercise, thanks to the grace of God in Jesus Christ. It is a sign of liberation and release rather than bondage and resignation."