I don't think having a concealed carry decal on a vehicle is a good idea.
It can advertise that a gun might be in that parked car, and make it a target of theft.
The car might be driven by someone else in the family who doesn't have a CCL.
The CCL driver might not be carrying that day.
Some people consider it an invasion of privacy and some consider it a sign of aggression.
It makes the driver a target of anti-gun nuts.
It contradicts the concept of "concealed."
During our CCL instruction, it was recommended that we not be careless in informing people that we have a CCL, and definitely not inform people that we're carrying. (It's different in open carry states but I don't live in an open carry state.)
It's another way that government can keep track of CCL people and collect a fee by regulating the issuing of the decals.
A mirror hang tag isn't a good idea either, for the above reasons plus:
Hang tags aren't supposed to be left hanging when the car is in motion (I know, drivers with disabled parking hang tags ignore that rule all the time). That means, if a CCL driver gets pulled over, he has to reach over and hang the tag. That's another movement.
Finally, the police aren't going to put faith in decals or hang tags. They know that the bad guys who could do violence are carrying their guns illegally, and they aren't going to inform them that they're armed.