I'm well on my way to 50k but I feel like my story has not plot and is quite bad. I read somewhere that NaNo is awful for quality of writing.
(How) can I improve the quality of writing during NaNoWriMo?
If you are following the daily wordcount rules, NaNoWriMo is explicitly not about quality. It's about committing to getting stuff on paper so you can work with it.
So many of us start a novel and never finish it, or never start it at all. Being part of the NaNoWriMo sprint gives you a concrete goal and a community to cheer you on.
There is, in fact, nothing wrong with having a crappy first draft. Most first drafts are crap. A first draft which you've written by aiming for a word count is very likely going to be a little extra crappy. This is fine. You now have your words on paper. You can see how your ideas did and didn't work. You have clay to make bricks with. You can throw out whole chapers and rewrite them because you saw how it didn't work the first time.
Stop worrying about "This is bad." Focus on "this is getting finished." You can edit at leisure.
It's important to understand what NaNoWriMo can and can't do.
In a way, NaNoWriMo is all about quantity, and not at all about quality. That's really important, because quantity is something a lot of writers really struggle with - sometimes without even realizing it.
Some of NaNoWriMo's big goals are:
These are the goals NaNoWriMo can offer you. If these are areas you want to work on, then NaNoWriMo is a great way to that (and to enjoy a ton of community support and encouragement while you're at it!). If that's not where you're having difficulty, if those aren't areas you want to spend effort to improve, then NaNoWriMo probably isn't the right tool for you.
Not necessarily, no. In fact, probably not. NaNoWriMo puts an emphasis on words, and on getting words down on paper. But words are merely a vehicle. What we call "writing" is actually about storytelling, not grammar and vocabulary. The relationship between words and story seems to be different for different writers. If the discipline of producing words has the effect of pulling story out of you, then NaNoWriMo may have some merit for you. But it is equally possible that the demand to produce words may cause your mind to disengage the story engine and simply start churning words. In that case, it is a waste of time.
Stories seem to flow out of some people as easily as breathing. They may lack nothing but the discipline and craft to write them down. For others, words may come naturally, but crafting story is a hard slog. Some seem to benefit from putting writing aside and building story as an outline or plan. Others can only capture story through words, but need to wait for story to come to them before they can write. Still others (the greatest number alas) have neither story nor art in them and can only spew words. Personally I can see NaNoWriMo being effective only for the first and last groups.
Story is the point, though, and if NaNoWriMo is not pulling story out of you, I would say stop and wait until you have a story before you write.