Why the film is intentionally boring/bad...
This movie is about Coogan/Brydon/Winterbottom (playing extreme versions of themselves) making a movie.
The movie they're making is an adaptation of a novel called 'The Life and Opinions of Tristam Shandy, Gentleman'
Details of the novel can be found here: http://en.wikipedia.org/wiki/The_Life_and_Opinions_of_Tristram_Shandy,_Gentleman
From what I've learnt, it seems this novel is regarded as quite an impossible story to re-tell in a movie, because of it's disfunctional structure.
In their blind stubbornness, the cast/crew believe they can still make it a success by struturing the movie as a comedic, narrated story.
As it turns out, no - their movie doesn't turn out well. And that's deliberate.
I suppose this type of movie would appeal to Monty Python fans.
With this knowledge, I still believe this movie is far from great, but it's perfectly watchable.