Games like Nightmare Pop!
Games like Nightmare Pop! include Hollow Knight, Hollow Knight: Silksong, LIMBO, Cuphead, and Super Meat Boy. We matched them by the tags players give Nightmare Pop! on Steam (Indie, Early Access, Platformer, and 2D): closest matches first, and among equally close ones the better-known games.

Hollow Knight
Shared: Platformer · Indie · 2D

Hollow Knight: Silksong
Shared: Indie · 2D · Platformer

LIMBO
Shared: Indie · Platformer · 2D

Cuphead
Shared: Platformer · Indie · 2D

Super Meat Boy
Shared: Platformer · Indie · 2D

Skul: The Hero Slayer
Shared: Indie · 2D · Platformer

Rogue Legacy
Shared: Indie · Platformer · 2D

Celeste
Shared: Indie · Platformer · 2D

Spelunky
Shared: Platformer · Indie · 2D

Shovel Knight: Treasure Trove
Shared: Platformer · Indie · 2D

Mark of the Ninja
Shared: Platformer · Indie · 2D

Geometry Dash
Shared: 2D · Platformer · Indie

Broforce
Shared: 2D · Platformer · Indie

Dust: An Elysian Tail
Shared: Platformer · Indie · 2D

BEEP
Shared: Indie · Platformer · 2D

Nine Sols
Shared: 2D · Platformer · Indie

Awesomenauts
Shared: Platformer · 2D · Indie

Rain World
Shared: Indie · Platformer · 2D

Out There Somewhere
Shared: Indie · Platformer · 2D

Running Through Russia
Shared: Indie · Early Access · Platformer

Defy Gravity Extended
Shared: Indie · Platformer · 2D

GRIS
Shared: Indie · Platformer · 2D

BlazBlue Entropy Effect
Shared: 2D · Platformer · Indie

Pizza Tower
Shared: Indie · Platformer · 2D