That's because his choice of inappropriate and disrespectful attire is a big message in and of itself:
- Trump is hopelessly gormless and crass. You won't get through to him with any amount of subtlety.
- He chose to be provocative. This was no accident. It's his way of saying "You can all go to hell, I'm better than you and I do what I want." which is something any diplomat who "makes a deal" with him should always bear in mind.
- He signals that his ego is bigger than the largest religion in the world, and focusing the attention on him is more important than a dead pontiff. Again, anyone dealing with him should bear that clearly in mind.