As the others have noted the pouring does not always work. Like half of the time just nothing but steam comes out, though it seems the customer doesn't care, the amount of each ingredient is apparently of no importance.
I like the general mixing idea a lot. It reminds me a bit of the old game Pizza Connection where you had to recreate existing pizza recipes by putting ingredients on the dough. So you could perhaps give a recipe and a picture and the players would have to guess the correct amount and place a fruit or some other decorations according to the picture seen. The closer you get to the original the better your rating.
The keyboard controls don't feel smooth or fitting to me. Since your score depends on how fast you are, being limited by the movement speed of the tray feels like a burden. You could instead make everything mouse controlled - allow the player to pick up and place the cup and move the tray around by mouse. But you'd have to be a bit careful as to not flip the cup over or spill its contents by moving too fast.