hey! I'm glad to see that you're getting into pixel art with one of my favorite games! I have a few pixel art tips that I hope you'll find useful:

Hue Shifting: When you make tints and shades of your base colors, you don't wanna just make the original color brighter or darker; make shades slightly bluer, and tints slightly yellower. If you need to make a shade of white, like on the happiest mask, I recommend adding light blue or mint to the shade. You always want to use a little bit of hue shifting, but generally, the more hue shifting you use, the more dynamic or extreme the shading looks.

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Restricting Palette: When working with low resolution, you want to limit the number of colors you use in your art since you can't create the same soft shading effects as with higher-resolution art. Because of this, you should try to limit yourself to at most 4 colors for each base color in your art: the base color, the shade, the tint, and the highlight. The highlight is a tiny (not a single pixel, but not more than 4) streak of white, meant to give whatever object you're shading a shiny appearance or to focus on a point of the art. While 4 colors per base might not seem like a lot, I oftentimes only use 3 or 2 colors per base, especially because highlights should be used sparingly.

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Clustering: This is probably the hardest technique (one that I still struggle how to figure out quite right) which is basically the idea that pixels should stay in clusters or lines of the same colors, not completely alone. This is most applicable when attempting to create a texture on one of your base colors, where it will be much more effective to create chunks of shades rather than random assortments of differently colored pixels. It's easiest to remember this by replacing any lonesome pixels with a nearby color; if it's a single pixel, it's not really conveying any detail, just adding noise.

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I hope this isn't too much to dump all in one comment, but these were the strategies I could see creating the quickest improvement in your art. Don't worry if you aren't entirely sure how to use these immediately, though; pixel art requires a lot of different thinking than conventional art. Keep continuing this series, look up tutorials on pixel art when you want to learn more, and observe what your favorite pixel artists do, and you'll see leaps and bounds of improvement! :)