Bathos is a figure of speech. It applies to a whole text, or a substantial part of it. (There is an associated adjective: bathetic.) Bathos is an anti-climax, usually so strongly marked that it becomes funny.
OED calls it "Ludicrous descent from the elevated to the commonplace in writing or speech; anticlimax", and goes on to define anticlimax as "The opposite of climax ... the addition of a particular which, instead of heightening the effect, suddenly lowers it or makes it ludicrous." There is an adjective anticlimactic. (See also climax, of which anticlimax is, obviously, the opposite.)