There's a green checkmark next to the seed icon if you've collected all the seeds in an area, same with the rocks.
Star #1 is for beating the game, star #2 is for getting all the items, star #3 is for buying all the clothes.
If you've beat the game, Snoozer will give you hints about where some of the "useless" items are.
Note that getting all the stars is just for completion, there's no hidden endings.