I believe that we need to respect the expected norms of any church we attend, If it is not the desire of the church that hats be worn, then out of respect for that church, we should comply. As I said if someone does not comply, it is not a sin. I agree that wearing a hat may not matter to God, but our attitudes do concern the Lord.
Of course, if we are looking to anoint a King, a hat is not important, but our respectful attitude toward others may be.