Mahler is not Schoenberg, and Mahler is not Webern, and Mahler is not Pierre fricking Boulez. He’s still solidly in the Common practice period, and his music is full of “vernacular music” catches, which make his music quite approachable. The marches, the dirges, the ländler, the lieder.
But Mahler isn’t Johann Strauss either (despite his half-hearted attempt in the Seventh). Mahler architects some pretty massive forms in his symphonies—far more ambitious than anything in the Classical period. Particularly after the first four symphonies, he puts on some heavy polyphony. And he goes through some prodigious emotional journeys. Not to mention that sometimes, he’s doing irony, and not singing to you what he means. (The finale of the 5th is a clear instance—a deliberate study in anticlimax; the finale of the 7th less so.)
I got into Mahler in my teens. I remember that the first time I heard a new symphony of Mahler’s, it would be a jumble of tunes. I needed to listen all the way through, several times, before the overall structure could coalesce in my head.
It’s superficially easy to get into; but to get it, I think, including getting the structures, needs several attentive listens.