Well it might not be a problem of topology, @dolores74 Could you provide a wireframe of your mouse?
Are you using any corrective hsapekeys?
In this kind of character and rig, it’s the go to for shoulder deformation. This could be fixed in few minutes.
Shoulder is the hardest part of the body to be rigged.
BTW, I’m currently recording the rigging course. And I’ve just achieved that before going to bed.
It is 100% bone based without any corrective shapekeys. A special something something I’m preparing 😉
