Also i'd consider breaking up the mass shapes abit more. As it is it looks like it's on a flat backing, like green pudding on a plate. I'd have a go at blocking out volumes again in layers so you can get a more 3 dimentional image.
If any of that makes sense. xD;;
Goodluck i love pattern dither when it turns out well but i've allways been scared to try it. So goodluck!

EDIT: I figured i probably wasn't very clear. So i did a quick and dirty (painted with a mouse in ms paint) edit to show what i meant about layering the clumps to give it a more 3D look.

You could certainly use this in a more cartoony style such as what you have going on, just needs some more depth!
