I don't think a lack of understanding irony is the problem here. Dilbert is being more sarcastic than ironic. Or did you mean to say "sarcasm" rather than "irony?" Sometimes people get those words confused, even when English *is* their mother tongue.
Coming from Denmark, apparently you are not aware that in the U.S. we also have grown up around not only irony but also plenty of sarcasm. (Ever see a Woody Allen film?). That said, there are many people from any number of countries whose families just didn't appreciate or didn't have the ability to understand either irony or sarcasm. Also, sometimes, people think they are good at it, when, in fact, they are not.
Also, this is not a case of a Dane trying to use irony on Americans. This is a case of an American picking a subject to mock that we usually don't find "mock-worthy." Maybe in a different context this could have been funnier. I wasn't even offended by this strip, I just didn't think it was that funny, unless there is some inside joke of which I am not aware.
Since some here seem to think that car restorers are crabby, we can try the joke with a substitute hobby. Restoring vintage clothing, antique dooknobs, upholstery, furniture, etc. The joke doesn't work then, either.
Do Dilbert and Wally know what a skilled (and lucky) car restorer can net on the right car restored properly?
And of course, all this from the man who spent an afternoon reading a book about beating a golf video game.