In 1991 Gerald Ratner wiped £500 million pounds off the value of his business following one speech after suggesting the merchandise his jewellery business sold was total crap. So what does this have to do with you?
Over the past year or so I have noticed a lot of up and coming authors being incredibly self-depricating. Phrases along the lines of "I've written some stuff but it's not very good." Why would you say that? It is all very well to be coy about your ability but you are essentially saying "Do not bother reading my work because you will not like it."
I get that it is a defence mechanism whereby you get the dig in before someone does it to you (hell, I've done it myself so you can call me a hypocrite). There is a very British tradition which suggests we should not be too brash or bold, that we should not tout our wares in public. And that can be fine to a degree. I personally think you should be promoting the value of your writing but I caveat that statement with having the self-awareness to know how good (or not) your own writing is (which could be a whole other blog post). However, whether your writing is wonderful or pathetic, the last thing you need to do is belittle yourself in public.
People take notice of what you say. Really, they do. Every time you post something online people are building a perception of you, your character and, if you post about your writing, your work. It is human nature to form opinions on everything and anything. That is how the world works for better or worse (which, of course, is my opinion). If you were to go into a restaurant and the chef told you the food they had cooked wasn't very good then would you buy double portions of their cuisine? I thought not. Why should people buy your short story/novella/novel then if you say similar things?
Be coy if that suits your personality. But be smart about it. Promote your work. Tell people you have a new story out. Don't shout it from the rooftops if it makes you uncomfortable. But let the reader decide if they like your writing or not rather than prejudicing their opinion. "Here's my new story, see what you think," is a lot more open than "It's not very good but have a read anyway." If they don't like the story at the end then that is fine; we all like different things. At least they will have taken a chance on you with an open mind and maybe, if they like what you have written, you'll come away with a new fan.