Everything has been mostly said already :)
The game is interesting, to work with music is rare enough to be noticed and welcome. Easier levels, ...
To give my part and give you my thoughts:
-I would make the move between bars much more easier (down to go under and up make you go through like a one way platform). Having to go all the way to change bar and the move out all the MallowMash to get them back in order is something you don't need when you try to get them at the right place at first haha. If you feel like it would remove too much of the puzzle part, maybe add points for linking a mash to its closest note?
-I would also add a hint when you leave a MashMallow on the right note? like if it turn green or different shades of green so you can see if you are right or not.
I feel like it would be a great game to learn notes, and music. Well done :)