I use pop rivets on sailboats and on sheet metal (e.g., ductwork), but like Derek warns, I would never use them on a cast plane frog.
Make sure the back end of the rivet is very firmly backed up by your anvil, and make sure you only hit the front end of the rivet. You should never hit the frog itself with either the hammer or the anvil.
If it is a brass frog, that is different. Send it back to Lie-Nielsen. They will fix it for free.