The experimental, boundary-breaking nature of Schoenberg’s works does fit in with other artistic works of the time, particularly James Joyce’s Ulysses, which had been published in a serialized format starting a few years prior to Pierrot Lunaire. The parallel between Schoenberg and Joyce is strong – Joyce smashed rules of sentence composition and plot in his works and took pride in the inscrutability of his writing, while Schoenberg abandoned all traditional musical structure with his shift to atonality and thought of himself as a pioneer of a new, greater form of music for the future.
I don’t think Schoenberg’s Pierrot Lunaire relates very well to the original text that Giraud wrote. The original feeling behind “The Moonfleck” and “Serenade” was humorous and tongue-in-cheek, and Schoenberg’s adaptations take the mood in the opposite direction, making it seem gloomy and suspenseful. This to me creates a disconnect between Giraud’s writing and Schoenberg’s scoring. Of course, it seems Schoenberg was perfectly aware of this disconnect, as he wrote the instructions for the singer to dissuade them from trying to express a feeling based on the lyrics and to instead stick with the mood of his own composition.
I would hesitate to classify Schoenberg’s music as Expressionist based on Pierrot Lunaire. This is because I didn’t get the impression that Schoenberg was actually trying to express a darkness in his own mood or psyche – it seemed like what he really cared about was experimenting and showing off his own compositional genius, and the mood of the music was more due to the atonality which was an aspect of that experimentation. Essentially, I don’t think Schoenberg should be considered an Expressionist because the feeling behind his work doesn’t come from a place of sincerity.