This is a three-way joke.
First and most importantly, in colloquial English, making an impressive mess of a task can be described as 'making a pig's ear of it'. This would not normally involve simply failing. The scenario would typically be one where you had laboured honestly and sincerely, so that it is hard to understand how you could have failed quite so thoroughly.
This is the kind of expression that would never be shouted. There would be a pause, possibly a deep breath, and the observer (imagine a flabbergasted teacher) would say something like, 'Well, we've made a bit of a pig's ear of this, haven't we?'
If this were in a film, we might easily pan from that disappointed face to see an eccentric construction that looks roughly like the thing it was supposed to be... except for all the treacle dripping off it (because you couldn't find the glue, but you did your best)... and then part of it drops off onto the workbench. But it must have taken you hours of genuine effort even to build it to that point.
(This, by the way, conceptually distinguishes the 'pig's ear' expression from the one about trying to make a silk purse from a sow's ear: that is about the foredoomed futility of trying to achieve something delicate, subtle and nuanced by using grossly unsuitable materials—see facsimile here, top of right-hand page. In the 'pig's ear' scenario, you had everything you needed and really could have succeeded: you just somehow managed to make a pretty inexplicable and catastrophic mess of it.)
Second, there is that stereotype of the Chinese being willing and able to cook and eat just about anything... with a little ingenuity in preparation required in particularly challenging areas.
And third, the pig itself is fairly often celebrated (in both east and west, I believe) as the animal of which pretty much every part is (sometimes with ingenious preparation) edible for humans.
The upshot of all this, in Pratchett's wordplay, is that the product in question might seem in some sense suspect or ramshackle, and perhaps even dangerous, but nevertheless be essentially fit for purpose. Instinctively, however, you might still be understandably wary of the culture and the individuals who came up with this bizarre approach.