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Are you reading a translation, perhaps? In my English version of the book, he does not scratch "KASPAR" on the coach, he scratches "MR PUNCH".
This is a dual-layered reference. First, Mr Punch refers to Punch and Judy, a puppet show that originated in Italy in the late 1600s but was also popular in the Victorian era.
The basic premise of Punch and Judy is that Punch punches his wife Judy a lot, along with everyone else he encounters. It is considered to be "low" or "slapstick" humour, and it's "funny" because it's puppet violence in the style of the Three Stooges.
Dodger would definitely have known about Punch and Judy. Charles Dickens had this to say about Mr. Punch:
In my opinion the street Punch is one of those extravagant reliefs from the realities of life which would lose its hold upon the people if it were made moral and instructive. I regard it as quite harmless in its influence, and as an outrageous joke which no one in existence would think of regarding as an incentive to any kind of action or as a model for any kind of conduct. It is possible, I think, that one secret source of pleasure very generally derived from this performance… is the satisfaction the spectator feels in the circumstance that likenesses of men and women can be so knocked about, without any pain or suffering.
The other layer to this is that Pratchett's friend Neil Gaiman wrote a graphic novel in 1994 called Mr. Punch, about a homicidal puppet.
Edit: I just realized there is another, more literal layer to this reference. A diplomat could be called a puppet, whose strings are pulled by their home government, and a puppet who hurts people would definitely be like Mr. Punch.
Scratching it on the diplomat's coach, which would go out in public, would be like scratching it on a car with a key - a public humiliation, along with property damage.