5th metacarpal broken, right hand. can i work with a mouse, would it affect healing?

Maybe: If the alignment is acceptable you may be placed in a cast then you can use a mouse in the cast because your index and long finger will be free. It may be more awkward than usual.

Maybe: It would be best if you wear a splint while using the mouse, otherwise you will tend to curve/roll the side of your hand over the edge of the mouse, and yes, that repeated flexion and un-flexion will delay or prevent proper bone healing and could win you a surgical pin.