YosemiteYogorockBlondelle,
I wouldn't say it's hard, or even difficult, but indeed, the trick to getting the
shaper's shawl is to have a fishing skill at or around level 100, and having the best rod in the game already (I believe it is not impossible with somewhat lower skill and a lesser rod, but I would not suggest one to go through such an ordeal, for even with my trusty Ebichu (or Ebisu), Mega Moglification, and a fishing skill of 107 without equipment bonuses doesn't make it too fun).
Fishing skill at around 30 and a little bit (or a lot) of gil may also bring one to the shawl.
I got one from about seven trades I think. That's only because I actually wanted a friend to get it first, then maybe I'd get one for myself. If I had, say, nine catches, I'd send the eight via delivery box to the friend, and trade in one myself because, really, there's no way I'd get it like that.
Well, the game obviously knows these things, so I did get it just like that, while the friend has traded in around one hundred (and counting).
The first case is mainly why you will often see fishing-related equipment on someone with the shawl as well; it comes from pretty much the bossest of boss fish after all (possibly the most annoying one, too), the matsui, er,
matsya.
I do not believe the shawl affects fishing skill gain, but I could be wrong, too. I do think it's not something we players can test...