Does your fireplace have a metal firebox that your stove is in or is it made of brick?

Fireplace is brick. Before/after attached.

Did you have your chimney flue inspected before installing the stove? Were you using your fireplace much before installing the stove? Does your flue pipe run all the way to the top of the chimney? If mice are entering through the chimney flue then that means that somewhere in the flue is an opening for them and that’s dangerous if your pipe doesn’t go to the top of the chimney and is sealed.

Steel wool might solve your problem if you can stuff it around the insulation. Mice won't chew through it.

@jrb451 - fireplace was not used before stove was installed. Yes the stove pipe goes all the way out the top of the chimney. Agreed, there must be openings in the chimney into the upper wall of the house as we are certain mice are finding their way in.

@Olychick we are using steel whool in smaller spaces throughout the house, but I think it would take quite a lot to seal up this space.

You could fashion a collar out of sheet metal to fit snugly around the stove pipe and to the walls of the chimney. Make it in two halves to install. Make a pattern and use that as a guide in cutting the metal.