I find much of the story compelling and certainly serious. The personal story and the storymode on dungeons can oftentimes be very serious. For example, Twilight Arbor and Sorrow’s Embrace are very personal dungeon storylines for Caithe and Zojja (respectively). Sure, there’s levity in the game, but honestly it’s there in the real world too. A story can be compelling and amusing at the same time.
As to Tybalt I thought he was one of the best characters in the personal storyline because he actually had character. A lot of people don’t have character in the storyline but every once in awhile you’ll encounter a fantastic character that keeps you invested in the story.
I thought the tone of Prophecies was a little too “we need to do this straight faced it’s our first game people need to take us seriously”. I found that tiresome that there was little to no character or levity whatsoever. It was nice to find the same level of storyline with the added bonus of levity here and there in the Factions, Nightfall, and EoTN storylines. I believe that same level of seriousness and levity has extended into Guild Wars 2.
If you don’t like their humor, that’s fine. But one way or another, Mad King Thorn’s jokes always slay the audience.