Can I use 12 awg ground copper for 14 awg electrical box wirings?
Should be fine. If it's just pigtails in the box it won't even affect your fill, IIRC. If you are running 12Ga ground wire (not just in-box pigtails) you do have to allow for it in box volume calculations, but it would rarely cause a problem since the equipment grounding conductor is only counted once for box fill.