This game has decent playability and enjoyability but it's suffering from numerous limitations. If I could summarise this review in one phrase it would be: this game needs polish. It's now time to explain what I mean by that statement.
I played through the first few levels and I admittedly got bored of them quickly. However, I played through the entire game so that I'll be able to give an honest and fair review not based merely on the first few levels. I was hoping to see a reasonable progression and more interesting levels with new things but I was terribly disappointed to say the least. The only real "enemy" you ever get are fireballs, and I don't think the fireballs even start giving you a tough time until level 46.
Anyway, I'll be more methodical now, listing things about the game that I think need work or were pretty bad:
1. The music. It's not very good and sometimes it doesn't loop well (levels 10-19). If you're going to make the music repetitive at least make it sound nice and not annoying.
Furthermore, when you fail a level and you decide to "Restart" there's a chance that the music would layer over itself and you'd have to refresh the webpage to get it acting normal again. Of course it would be all good until you fail another level and you have to restart it. You'll have the pleasure of listening to overlapping cacophonous musical passages (a.k.a. noise) once more. This needs a fix.
2. Upgrades. Yes the upgrades can be rather nice, except they don't function perfectly. For example, in the later stages of the game pressing "X" stopped working. The player could no longer balloon himself to float a little higher. I decided to check the shop if maybe the game reset the bought status of the balloons and strangely enough the third balloon icon was faded out apparently indicating I haven't bought it yet even though I was sure that I've maxed out the balloon upgrade already. So to make sure that I really have maxed it out I pressed the button for purchasing but it's not buying, probably because it's already registered as maxed out while at the same time it isn't. Anyway, the balloons were no longer working either way.
Another problem was that the game also stopped registering my usage of the air-jump upgrade. In the first few stages a boot icon would disappear if I used the air-jump upgrade, but during the final few levels no such change is present, so even if I've air-jumped 3 times already 3 boots would still be there. The only things disappearing are the hearts and the sword (of course the balloons are not disappearing because the game won't even let me use them).
3. Coins. Collecting coins after you have bought all the upgrades is pretty pointless. You'd probably be able to buy the upgrades early into the game. It would have been nice to have some purchasable things that cannot be maxed out, e.g. one-time-use items.
4. Level design. All the levels are very bland with only a few enemy types present. However the only enemy that can damage you is the fireball. Every other enemy seems to be variations of floating stepping stones. It's very disappointing how the levels don't change all that much and to add to this the objective never changes! That's correct, every single level you have to kill 100 things! This gets old really easily. As for the levels themselves the environment only changes in levels 10, 20, 30 and 40. Levels 40-50 have the same background, the only difference with level 50 is that it is a kind of boss fight. You might think the boss fight would have a different objective, but no, you still have to kill 100 things. How annoyingly boring.
Another issue I have with the levels is the scattered difficulty. Finishing most levels probably have more to do with luck than with skill. The more "opponents" there are and the less spread out they are, the easier it would be to finish a level. But of course opponents are randomly generated, hence why I finished level 47 faster than I finished level 46, I just happened to get lucky that level 47 had more "opponents" and less homing-missile-like fireballs than level 46. Difficulty in this game is only increased with the frequency of appearance of fireballs and scarcity of slashable opponents.
There's not enough progression. It's the same thing for every level. Finishing the entire game is just a pointless slog that's not even worth your time. Cutting down the levels or at least making them more interesting by adding more elements and more challenges would make the game more enjoyable and less arduous or boredom-inducing.
5. User interface/menus/controls. In the levels menu, if you have bought all the upgrades, they would overlay onto the level selection. This is bad design. The upgrades should not appear on the screen until the player starts a level.
There's also no pause or restart button. Yes you could easily pause/restart at the same time by dying but what if you don't want to die?
There's no button to go back to the main menu. While there's not really anything to this game other than the levels, not having a button to go back to the main menu is kind of like opening a book and ripping out its front cover so that the new front cover is the table of contents.
Why Z, X and C? Why can't you just have another click for the air-jump? X and C I don't have too much of a problem with though. Also how about an alternative control system? Instead of using mouse/trackpad use arrow keys for movement and spacebar for jump/air-jump then X and C for what they were before. Mouse control is fine though, the controls are probably the okay-est part of your game. That's saying something.
6. The whole game. I did try to give your game a chance but it's broken and overall not very fun or interesting . It could be good but it needs a stuffton of improvement.

Nice use of a very simple mechanic in a really engaging way. The game's fun to assets ratio is very good. It reminds me of the scene in FF7:AC when Cloud jumps super high to fight Bahamut.

It's an ok game overal but it could use more upgrades and it should have less levels with more veriaty in between them

There needs to be some sense of progression. I got bored at the fourth level of the exact same thing. Kid jumping into the sky, trying to kill 100 bats. No change in scenery, he's not getting any higher up in the air, nothing. Not gonna play 50 levels of that.

good fun, easy to play on a trackpad which is a major bonus for laptop users.
like the simple gameplay and controls, makes for a quick fun game. the levels would keep me coming back.