The mood of the book is generally light and humorous, even though it's making a number of serious points. There's something amusing about someone like Wallace always telling the truth, irrespective of the consequences. It's not very pleasant to be called a liar, but we all do it; sometimes it's necessary, especially when we want to spare someone's feelings.
But Wallace doesn't do that. He doesn't mean any harm, of course; he's not being gratuitously nasty or unpleasant in criticizing either the book or the play. It's just that telling the truth is so incredibly important to him. But what's a matter of principle to Wallace is a source of amusement for the reader. When Wallace gives his brutally frank opinion on the literary masterpiece that is Old Shep, My Pal—"…the dog always dies. Go to the library and pick out a book with an award sticker and a dog on the cover. Trust me, that dog is going down"—it's hard to stifle a giggle at Mr. Fogleman's evident horror and discomfort.
But the humor conveys a serious message: sometimes it's necessary to tell the pure, unvarnished truth, even if it offends other people.