Firstly, I'm in love with this demo! The music and the art is so catchy and charming! I for one can't wait for a full release!
Second, there's a couple of bugs I think you ought to know about: the most important of which is that when remapping your controller, in order to remap specifically the buttons which are both face-buttons and the "select" and "back" buttons it forces you to clear the buttons which are start and select because it causes a conflict (despite the fact that those settings are technically conflicting by default).
The other bug I found was that if your status is "lo-fi" at the very end of Poppin' Grease in quick-play mode all the background chatter of dissatisfied customers keeps going into the rank screen and doesn't stop until you go back to the menu (which was admittedly amusing).
These bugs aren't so bad that you'd need to fix the demo specifically so long as they're not in the full release. All the other notes I have are really minor things (like how the score meter at the end of each song fills really slowly and is unskipable, and how an audio indicator for when it's your turn to sing would be helpful).
Really looking forward to seeing development updates and eventually a full release!!! <3