What does 痛車 Itasha mean?

An itasha is a car decorated with anime, manga or game characters, usually across large sections of the bodywork. The name means painful car, and who first applied it is disputed.

Whose pain

痛い (itai) means painful, and in casual use it also means cringeworthy or embarrassing, in the way English says a thing is painful to watch. Attached to 車 (sha), car, the compound describes a vehicle that hurts to look at.

Whether the name began as outsiders' mockery or as the owners' own joke is unclear, but owners use the word about their own cars, acknowledging that the thing is embarrassing, that it embarrasses passengers, and that the money spent is painful in its own right. Self-deprecation is doing the work.

The older pun

There is a second layer. In the 1980s, when imported cars were a conspicuous luxury, a shortened slang term for an Italian import sounded near-identical to this word.

So the modern coinage lands on a phrase that used to mean an expensive foreign sports car and now means a hatchback with a character printed across the doors. That is a considerably better joke than the surface reading.

Why it belongs beside doujinshi

Because it is the same culture in a different material. Doujinshi puts fan work into print and sells it hand to hand at conventions; this puts fan work onto a vehicle and parks it outside them.

Both are amateur production aimed at other enthusiasts rather than at a general audience, both involve real expense borne by the maker, and both are displayed in person at gatherings rather than distributed.

What English lacks

A name for elaborate fan decoration that is affectionate about its own excess. English has terms for customized vehicles and for fan display, and none of them carries the built-in wince.

The wince is the point. A subculture that adopts a name for its own practice from the embarrassment it causes has said something about itself that no neutral term would convey.