Only Mono sound for 2D game?

There’s any reason to put stereo sound in a 2d game where I does’nt need the effect of hearing differently in both ears? I’m making a mobile game and mono are weight less…

Yes, actually, some 2D games use stereo sound for various reasons. It lets headphone users more accurately predict, for example, where a boss off-screen is about to strike. Or, it sounds different when the player manages to move away from the camera. One example is Terraria, where the audio listener is attached to the player, so as you walk to the edge of the world in the ocean, many of the sounds on the screen become harder to hear.

But if you don’t need stereo sound in your game (or just don’t want it), then don’t use it. Set all your audio sources to avoid it. It is, I believe, slightly more performant, but only slightly. You’re better off worrying about your scripts, rendering, and physics.