It is entirely unnnatural to fire a gun using your thumb.
OTOH-if you’re going to plant a gun its the easiest digit to access.
Curious - what did ABC News have to say about this picture?
No, it is entirely unnatural to shoot yourself in the mouth. But when one does decide to do it, the thumb as a trigger finger is much more natural. Pretend your holding a gun. Now, put your right thumb on the trigger and put it in your mouth using your left hand to brace the back of the right hand and the gun. Simple and much more natural than using your forefinger and sticking your elbow way out in front of you or turning the gun upside down.
The thumb would be used to fire the weapon if you were aiming it into your own mouth. If you were going to shoot yourself by sucking on the barrel how would you pull the trigger with your index finger?