Tip: Use the Piggy Bank

This game gives you the same number of stars whether you clear a stage or not, but the amount you get decreases over time (always single digits). You can unlock the piggy bank by watching a few ads and get stars from it. The piggy bank actually gives quite a lot of stars, and you can unlock it once every hour. If you want to get more stars faster, I recommend diligently watching ads to unlock the piggy bank.