First off, great to see a game using PICO8! :)
The idea/story here is so original, I love it. The pixel art intro of the sheepdog with the dead shepherd (so sad!) is a great way to add a little bit of story to the game - I really care for the dog and its mission haha.
The gameplay is just as original and yet so simple to understand. After a few barks I got the hang of it (bark radius, sheep trajectory). The graphics are clear and have a cute vibe which I really enjoyed. The sfx are perfect, I like the mission complete jingle 🎶
You even included a map for selecting levels 🙌🏾 I really don't have much negative feedback except maybe the sheep pen could stand out a bit more (maybe with a flag or something?).
I've saved this game to play later with some tea :) Overall, amazing job!