Get a book that teaches you how to draw manga...it's actually very easy to get the hang of. I got a book a few months ago and only went through it once and got the styles down easily enough, and I typically have a lot of trouble drawing people. As for your question, I honestly do not think you can learn/improve something like drawing in a lucid dream, however if you create the intention and attempt to practice it in lucid dreams, it's possible it might somehow carry over to your waking state. But things like drawing, reading, etc. are not easy to do within a dream. It's just too tricksy. There's really no short cut to becoming a better artist, practice practice practice.