Welcome to the MongoDB Developer Community Forums @Mohammed_Sahil !
mongomirror is a tool for migrating an existing replica set to a MongoDB Atlas replica set so it has some assumptions about the destination (for example, that this will be a replica set).
I recommend using mongomirror if your destination is an Altas cluster, but for migrating to (or from) a more general destination I suggest using MongoPush.
Regards,
Stennie