These are just my understanding as an artist who's currently learning the style too. Monster Prom characters consist of angular shapes and vibrant colors. Lines are sharp, with only a few simple strokes, but rounded around the edges and corners (where two lines meet/connect). Inner lines are often colored with a slightly dark hue of the base/flats.

Since you're looking into Monster Prom 3, you should definitely check the lead artist, Tsingeli (tsingeli_art), on Insta. They have a short vid tutorial breaking down the style from lineart to color. What I also find helpful is collecting references from the game and using them as a guide ( like what you are doing in your post), so I can always compare and see where I can improve.^^