Hurry, hurry, I feel my temperature rising...
The [Wondrous] Boy-Horn Songs: Is anyone else just not that into these? I mean, yeah, there are gems, but I can't shake the sense that there's a bunch of filler too. And I like Mahler's songs! Hook me up with the [Lieder] of the Earth or the Dead Kid Songs and I'm in heaven. But the [Wunderhorn-Lieder]? So sue me...so much of it just feels like Mahler phoning it in.
This post from Wellsung deals with Des knaben Wunderhorn in Chicago. My now-annual Mahler pilgrimage to Chicago happens in late November, when I travel to see Pierre Boulez conduct Mahler's 7th. I can hardly wait. His Cleveland 7th on DGG is really quite excellent, though recently eclipsed by Barenboim's Staatskapelle Berlin disc on Warner Classics.
As to the matter at hand, the Wunderhorn-Lieder make appearances in Mahler beyond his Liederkreis devoted to the "Youth's Magic Horn." The 2nd, 3rd, and 4th all have songs from the set. The contralto solo, "O röschen Rot," from the 2nd is a Wunderhorn text. The 3rd has that "Bimm, bamm" tintinnabulation that leads into, "Es sungen drei Engel." And the 4th has the song, "Wir geniessen die himmlische Freuden." Mahler obviously drew a lot of inspiration from the Wunderhorn texts, and used them in most of his symphonies that include voices (excluding Das Lied von der Erde, which I tend to put in a class of its own, neither Liederkreis nor symphony). Of course, he also used Klopstock, Hrabanus Maurus, and Goethe in those symphonies. In any event, he might be phoning it in, but there's no denying how important the "Youth's Magic Horn" was to Mahler.
With that pedantry aside, Mahler composed a lot of Lieder: the Kindertotenlieder, the Rückert-Lieder, the Lieder eines fahrenden Gesellen, the Lieder aus Des knaben Wunderhorn, Das Lied von der Erde, and the stuff in the individual symphonies that include it. Having heard all of it, I can say it ranges from merely very good to really quite excellent. But, Mahler was not Robert Schumann or Franz Schubert. The notion of a man (or woman) singing with only a piano for backing seems a little alien to the composer whose sprawling symphonies attempted to encompass "life, the universe, everything" (42, by the way).
Mahler was a genius, in any sense of the word, but he was not a chamber composer. There is a difference.