Games like Happy Game
Games like Happy Game include Little Nightmares, LIMBO, Tiny Bunny, Sally Face - Episode One, and Little Nightmares II. We matched them by the tags players give Happy Game on Steam (Horror, Psychedelic, Cute, Dark, and Atmospheric): closest matches first, and among equally close ones the better-known games.

Little Nightmares
Shared: Horror · Atmospheric · Dark

LIMBO
Shared: Indie · Dark · Atmospheric

Tiny Bunny
Shared: Horror · Dark · Atmospheric

Sally Face - Episode One
Shared: Horror · Dark · Indie

Little Nightmares II
Shared: Horror · Dark · Atmospheric

Fran Bow
Shared: Horror · Dark · Indie

INSIDE
Shared: Horror · Dark · Atmospheric

Little Misfortune
Shared: Indie · Horror · Cute

DISTRAINT: Deluxe Edition
Shared: Horror · Indie · Atmospheric

Layers of Fear (2016)
Shared: Horror · Atmospheric · Indie

Outlast
Shared: Horror · Atmospheric · Dark

MiSide
Shared: Cute · Horror · Atmospheric

Stray
Shared: Atmospheric · Cute · Singleplayer

No, I'm not a Human
Shared: Horror · Indie · Atmospheric

OneShot
Shared: Cute · Indie · Atmospheric

Rusty Lake Paradise
Shared: Horror · Indie · Atmospheric

Mouthwashing
Shared: Horror · Atmospheric · Indie

Hellblade: Senua's Sacrifice
Shared: Atmospheric · Horror · Dark

Poppy Playtime
Shared: Horror · Singleplayer · Dark

The Room
Shared: Atmospheric · Horror · Singleplayer

Slime Rancher
Shared: Cute · Singleplayer · Adventure

Detention
Shared: Horror · Indie · Atmospheric

SOMA
Shared: Horror · Atmospheric · Dark

Five Nights at Freddy's
Shared: Horror · Singleplayer · Indie