An interesting question; to the degree that literature takes itself seriously, those who encounter it tend to take it seriously too, provided they can understand it.
On the other hand, a lot of people would say a work like 'Finnegans Wake' is nothing more than nonsense.
should anything that takes itself seriously be taken seriously?
is it a prerequisite that something be understood before it can be taken seriously?
Good point...maybe a distinction should be made between literature that takes itself seriously (or too seriously) and literature that recognizes both the absurdity and the solemnity of the human condition. The stories of Chekhov, for instance, can be taken seriously because they don't take themselves too seriously; they are not sentimental, but neither are they guided by an intelligence that is purposefully unfeeling.
I'm not so much asking which kind of literature should be taken seriously, simply whether any of it ought to be. Let's take all literature (fiction) ever written and throw it in a box, why should I take that box seriously?
I guess there is no purely unobjectionable reason it should be taken seriously. Maybe it depends on whether we believe in the sincerity of literature as a gesture, as an attempt on the part of the writer to represent something true about the human experience, and in so doing change us. Whether the writer succeeds in this attempt is another question, but the attempt itself might be enough to warrant our attention.