Literary anticlimax (6)
I believe the answer is:
bathos
(Other definitions for bathos that I've seen before include "Ludicrous descent of mood in writing" , "Change from serious to trivial (in literature)" , "Unintentional slip from serious to trivial mood" , "Unintentional lowering of tone" , "Anticlimax by unintentional shift from the sublime to the trivial" .)