As a thesis statement, you should be addressing Faulkner\'s main point for this novel. Faulkner is saying there may be no more creative moment for humans than when we rationalize our thoughts, words and actions.
Every character except for Jewel is consumed in a deep, developed personal lie. They are all appearing to heroically fulfill the dying wish of the family matriarch. They are all using this moment of grief as a means to a personal and selfish end. Darl is mentally torturing Jewel. Dewey Dell is desperate to get to town for an abortion. Anse needs to replace his dead wife. Cash is trying to win his dead mother\'s approval by building a coffin she will enjoy. Even Vardaman is conned into seeing toys in a store front that he will never own.
The horrible beauty of this dark novel is that only after the characters are fully fleshed out, there is no way to convince them that they are wrong. The scene of Dewey Dell fanning her dying mother appears to be like a caring daughter keeping a sweet vigil. She is fanning her so hard she is making it hard for Addie to breathe and she will konw first when Addie expires. It is grotesque, creative and hypnotic.
There are many themes touched upon in this novel. Alienation, loneliness, death, insanity and sanity, identity, the importance of language, love and passion are just a few. You could focus on any one of these themes to come up with a great thesis statement. For example, you could use this one: Faulkner uses language and a character's use of language to help the audience identify with each of the characters in As I Lay Dying. Your thesis should be a statement which you can prove with reasons/points from the novel. For the language one, you can use Addie's distrust of language, and Jewel's inclination to action rather than language. Darl, Vardeman, and Dewey Dell also have various language issues.