If you want to get a three star rating on a level you're going to need to grow objects, kill enemies and put out fires. Doing so will fill up the meter in the top-right hand of the screen. Max it out completely and you'll walk away with the maximum rating.
To gather more rain you're going to need to gather the smaller clouds that are littered around the landscape.
Forests are great for generating new clouds, so take every opportunity to grow a new bit of woodland.
Position Kumo in the center of the screen with a finger push that way you'll always be able to see if the dark clouds are coming.