The question is good, even though I'm not sure that I would say that Eliot's long poem indeed deals with "the disorder and decay of modern civilization." Eliot doesn't seem to me particularly concerned with morality (which is what I thnk of, perhaps wrongly, when I hear the phrase "disorder and decay"). Rather, I might go with the terms used in the themes section of the enotes study guide for The Waste Land: Disillusionment and Restoration (or rebirth). The poem's images develop these two central themes. Characters in the poem engage in joyless sex, the ground is not fertile and nothing grows, etc.
The text of the poem, too, may certainly appear to be in complete disorder. Song lyrics (from high- and low-brow songs) are presented alongside references to printed texts from high and popular culture (there's even at least one refernce to the novel Dracula, see the third length below). The textual fragments that make up the poem -- "These fragments I have shored up against my ruin," says the speaker in the poem -- reflect a fragmented, post-WWI society.