Hey! Great effort on the game! My thoughts on each category:
Overall fun:
While I can imagine people having fun on this type of game as it is in the same thread of extremely popular games like Tetris, unfortunately it's not my type of game (I also don't like Tetris, blasphemy I know!). I will try and compare it to Tetris for how fun I think it is, but take it with a pinch of salt given my personal preferences.
It has a simple mechanic of trying to add together 3 numbers to make a number. However, the time you get in between blocks falling is extremely fast for a new starter to the game. I would have expected on campaign mode for it to start out very slowly and maybe get quicker, like Tetris? I can imagine that when you are practiced at the game it, the fast pace is what you want. However, as a beginner, having to see the number you are trying to make, then see the next number coming, the see where you cursor is, then look at the board and see where the number would fit, then decide where to put the number, then move the cursor to the location, all before the couple of seconds you get is quite allot. I found myself starting a game, and before I realised what was going on, I had a large tower in the middle. I also don't like the was you select a row, then cannot change while the block is moving. I feel like this is a vital mechanic of games like Tetris, as it adds tension but also delays decisions for some moves, while allowing a quick reaction for others, helping the pacing of the game.
UI:
The UI in menu seemed to fit the casual theme of the game, yet it also seemed a little to basic. While I did like the buttons expanding on selections, I feel like every action took too many clicks. I would prefer each button be one press, which spawns an animation, which then fires the action of the button.
As for the in game UI, because the pace of the game is very quick, the parts of the game you need to see quickly (the next number, the number you are making, the time before next fall, and the selection row) need to stand out allot more. For instance, instead of an arrow showing the selected row, you could highlight the selected row with a nice bloom effect. This would also help with the difficulty without having to slow down the game, as it would be allot more obvious where the block was going to fall as you wouldn't have to look at the cursor, the follow the row down to the bottom.
Art:
The art theme was necessarily basic in a good way. You don't want everything drawing attention to itself because allot of focus needs to be on the current objectives. However, I didn't really like the background. I think having the blocks moving in the background adds to the confusion and complexity, when really the user is just supposed to be focusing on the game. I think a more abstract, washed out, slower background could work better, and not draw attention to the game allot better.
Sound/Music:
The sound effects where really nice. I really liked the sound feedback of interacting with the UI. As for the music, even though there we settings for music, there was no music playing in my game? I'm on Android btw.
Anyway, great effort on the game!