If you are trying to use a controller on mobile, some players have found they can manually map a "Crouch" action to a button if the game recognizes the input, typically defaulting to the key equivalent on PC.
: Your character will automatically crouch when you approach a vent, low tunnel, or any area that requires a lower height to pass through. how to crouch in poppy playtime chapter 2 mobile
The mobile port uses a collision detection system. If you are tilting your camera (right thumb) to look down at your shoes, the game thinks you are trying to look at the obstacle, not pass through it. If you are trying to use a controller