Sometimes, it seems to me, writers don't really believe that their plot and characters and settings and ideas are strong enough to hold up the weight of a story.
And so they add stuff, often jokey stuff. They make metacomments in the narrative to make clear that they know a plot development is implausible. They add unnecessary jokes because (I speculate) they feel like the serious core of the story is insufficient to stand on its own. They write lovely metaphors and then they mock them so readers won't think the author actually thought those metaphors were good. They write a powerful and moving scene, and then add something wacky and silly (mismatching the tone of the rest of the scene or story) for fear of sounding pompous or pretentious. In short, they undermine the stuff they're doing well, by not having enough confidence in it to let it do its job.
If you find yourself in this situation, I have two pieces of advice:
First, trust the strength of your material. If you've got a good story, it doesn't need those embellishments. It doesn't need disclaimers or extraneous jokes. (See below for more about jokes.) Focus on the story you're trying to tell and the characters who are part of that story and the situations they find themselves in and the tone you want the story to have, and other things that form the core of what you're doing.
(It's not that all embellishments are bad; it's that embellishments that work against what you're trying to do with the story are self-defeating.)
Second, if your material isn't strong enough to build a story on, then adding a lot of distracting flourishes and jokes and fireworks and adornments and look-over-here misdirection will not make it strong enough to build a story on. At best, it'll distract the reader from the weakness at the core of the story. At worst, it'll distract the reader from the real strength at the core of the story.
So if you're worried that your story isn't strong enough to stand on its own, then by all means reinforce the foundation, shore up the weak timbers, find more durable building materials, or even take apart what you've got so far and build it again on a new and stronger foundation.
But if you've got an old stone building that's in danger of falling over, then adding bright fuchsia curtains and a windmill and a high-tech radio tower and a helipad and a sign saying “The builder is aware that the stone we used for this is a lousy building material” will not make the building any stronger.
And if that stone building isn't actually in danger of falling over, then all those additions may make it hard for visitors to appreciate the solid underlying architecture.
Some side comments:
I don't think I made up the phrase “trust the strength of your material,” but I'm not sure where I got it. I've been occasionally using it (and similar phrases) in comments on submissions since late 2000 if not before.
The web pages I've seen use such phrases are generally discussing the overuse of explanations and descriptions. That's also a form of not trusting your material, but not quite the same thing I'm focusing on here. Those web pages also discuss a related idea: not trusting your readers to understand without a lot of extra explanations and descriptions. (I have trouble with that in my own writing.)
I suspect some of you are saying “Of course it's okay to put jokes in stories!” As usual, this entry is more narrowly focused than it may sound. I'm certainly not saying stories shouldn't contain jokes. I'm saying that if you're throwing in jokes (or disclaimers, or other stuff) that don't match the tone of the story, primarily because you're worried that the story isn't strong enough without them, and if those jokes (or whatever) distract from or otherwise lessen the impact of the core of your story, then you should consider leaving out the stuff that's undermining the good things about your story.
Sometimes a little handwavey distraction is reasonable. Some writing instructor (I forget who) once told me, “Skate quickly over thin ice”; and sure, if you've got one patch of thin ice in your story and if you can get away with it, that's great advice. But I would advise against trying to perform a juggling act on a unicycle while doing that skating—especially if the ice is really much thicker than you're worried that it might be.
(Wrote this entry in March of 2010, but didn't get around to posting it ’til now.)