Really glad to hear you enjoyed the game!! Spent a lot of time working on the polish so I'm glad it paid off!
And yep, you're definitely right that the win conditions aren't explained very well in the game. If we keep working on it I'm sure we'll add some more details in.
Every happy goblin that goes through the exit increases the hidden background score by 1, and every angry goblin decreases it by 1. After we designed a puzzle, we would check what the maximum and minimum possible scores are for a level, and those would be used as the thresholds for 0 and 5 stars. This is why on some levels you actually start with more than 0 stars, since it's possible to lower your score in the level. I was also tempted to try design a semi-hidden secondary objective where you can try to get the lowest score in each level, which is why you may have seen an "Ultra Fail" endscreen if you finished a level with 0 stars. Didn't quite finish implementing that though, especially since it's pretty easy to get a low score on some levels. Though in others it's actually harder to get a low score than a high one!
I was a little bit uncertain about adding the portal maze into the game for the exact reason you mentioned, isn't really a puzzle as much as it is a guessing game. Ended up leaving it in after one of my girlfriends said she enjoyed it though.
The cloning mechanic was something I wasn't even sure I was going to have time to add in, but after seeing the positive response all of the cloning levels have received, I'm very glad I managed to get it done! It does open up a remarkable amount of possible puzzle design.
Thanks for the long comment, I really appreciate it a lot, I'm sure the others in my team will too!