Mirrors allow us to have content protected and out of Reddit’s control. If Reddit decides to tighten up their grip on the API even more, the mirrored content will be already safe from their hands.
I think you are confusing people here by saying mirror. They think about it as another frontend.
I suggest to use Matrix terms. Here what you have would be one-way bridging
One-way bridging is rare, but can be used to represent a bridge that is bridging from the remote system into matrix. This is common when the remote system does not permit message posting, or is simply not capable of handling posting outside their system. The users bridged from the remote system often appear as virtual users in matrix, as is the case with matrix-appservice-instagram.