All the things you mention are player errors. Do not blame the Nintendo Wii or Super Mario Galaxy.
1. Get better? I've yet to find a single complaint on this aspect, until now. You just have to flick the thing and Mario instantaneously spins on screen. Its not that difficult.
2. Check your Wii settings, you need to set the pointer according to how far away you are from the screen. Also, sitting at an extreme angle can cause this.