A perfect example of humor in Death in Venice seems to occur in the very first sentence, where we find out that Gustav Aschenbach changed his name to Gustav von Aschenbach on or around his 50th birthday.  "von" is a preposition that when used as part of a family name indicates nobility, according to Wikipedia, and the text seems worded to suggest that Aschenbach wanted to make himself sound (or perhaps feel) more important than he really is by beginning to use that title.  The narrator continues to mock Aschenbach throughout Chapter 2, pointing out how he considers himself to be an artist among artists, the best of the best, while believing himself to be meek and praising the way that his works became "conventionally polished... even formulaic" as he became older.  Formulaic is certainly not a thing one wants art to be.

Another example of humor can also be found in the first paragraph, as the majority of the first paragraph says, in excessively flowery and eloquent language (to the extent of quoting Cicero and using a Latin phrase) that Aschenbach was too restless to nap.