I like the game's main concept, which is mixing the sounds of various monsters on the stage to try to make cool and interesting beats with them. The unique looks and sounds of each monster piqued my interest and that was why I ended up buying the game. This is where the game shines and holds all, if not most, of its value.
The other part of this game that I'm more conflicted about is the hatching mechanic at the lab. I understand this is here to prolong the gameplay and to make receiving new monsters more exciting, but it doesn't feel rewarding. The amount of time you end up waiting is long enough to make me put the game away and short enough to make receiving the new monster feel not that exciting.
Something I think could have worked better for introducing new monsters would have a random set of monsters appear each day on a separate stage, these could be new ones or ones you already have in your collection. Once you have seen them at least once and have them in your collection, you could then use them like you currently do on the stage. This I think would add more excitement to open the game each day to see what new monsters you might find, and the randomized set could be used as a challenge/constraint for the player each day.
Regardless, I still really like the game and can't wait to see what else you make!