Nathan Bransford posted a blog on this, so if you want to read my opinion on it, I suggest reading this first:
In any case, for the most part, I can see how the Dunning-Kruger Effect would hold some truth to it. However, it's my belief that those who think they TRULY suck wouldn't be getting better at all. In fact, I don't think they'd be doing it at all. I suck/hate drawing (realism drawing); therefore, why would I want to improve? Deep down, people who keep writing don't actually believe they suck, but keep telling themselves that because they are embarassed to think otherwise, to think they can actually improve and are improving. People who keep trying are just putting on this illusion that they think they suck. I'm certain, deep down, they are just uncertain and would rather not say whether or not they suck.
That's how I am. I know I'm good, but when I do a piece of writing, I put my mindset on neutral so I can become open to criticism. I do not think that the piece sucks, but I also do not think the piece is good. I let my beta readers decide all that for me.
Does this mean I don't have my self-doubt moments? No, but it's rare that I do doubt myself. I've had them before, about once last year, but it quickly passed. I can say from experience, though, that during that self-doubt moment, I didn't want to write at all. I just didn't believe I could do it well.
I suppose it differs for everyone, though. If I believe I suck, I'm not going to bother getting better at it. But if I go in with confidence and a neutral mindset about my abilities, I do get better at it. I also think I just have to love what I'm doing. I suck at realism drawing, and I hate it. I think there's a common connection there. Yet, I love writing, and I'm getting better at it, and I know I don't suck.
There is a difference between confidence and arrogance, which Dunning-Kruger fails to mention. I think if you want to get good, you absolutely need to have confidence. Of course, if you think you're great and don't need to improve, you probably aren't that good.
I'm confident, not arrogant. Don't be afraid to have confidence in what you do. Just realize that you can keep improving and getting better. I'm certain Stephen King doesn't think he sucks, but I also believe he's aware he can keep improving.