Okay, so a few things: Rather than map out the flat background, I just made a low-poly UV map and painted the grass and clouds. This saved space, and since the map was 521x512 it gave the background elements that pixelated look. The stripes on the grass were made using Texture Paint > Stroke > Stroke Method Line.

 

Instead of worrying about lighting, I changed all of the materials to emissions. (Thank you, Lumchii!) Then, went to render properties > Color Management > Look Medium-high contrast and View Transform Standard. This gave them the bright, flat colors of low-poly pixel art.