Here are some crits. First off, your lines indeed need some smoothing. Your curves in your line look jagged.
Here's what I mean:
The second is what you are doing, and the first is how it should be done. It will make your art look much nicer, especially seeing you're going for a bit Wind waker esque cartoony style, which involves curles. And curles obviously look better smooth. If you can't get it right, try resizing your curl/curve.
My second point of critique would be shading. You might've read Tsugumo's tutorial, if not, read it (
http://www.geocities.com/skulkraken2002/) . Which shows some examples of a tree.
You can use kind of a spherical form of the tree, as the spheres would show all bunches of the leaves. This means you can simply use basic spherical shading and clean it up.
Like this:
Off course, you can tweak the shape however much you want. (Don't look at the trunk, by the way)
And you can stylize it as much as you like.
Also, your dithering seems to be very jagged, and there is too much dithering overall, you can simply use your colours better and get
rid of the dithering.
Well, this is my first kind off in depth critique, go easy on me
