I've played the Whimsyshire level a few times but only saw the pinata once near the exit stone. Is the chance of it spawning random? If it does, does it always show up near the exit stone?
No it does not.
I've run 3 full clears of Whimsyshire, and never seen the pinata once.
Since the Pinata is just the Whimsyshire version of a Resplendent Chest, I would assume that it has a relatively low chance to spawn.
I would assume however, that it will always spawn somewhere near the exit stone, as most resplendent chests in dungeons do so.
The Pinata can spawn anywhere. On my second last run it had spawned just inside the entrance point. On my last run I didn't find one at all. I am currently at 40% for spawning but have only done 10 runs so i may have been lucky/unlucky or missed it on occasion. It doesn't seem to spawn every time however.