It's not bad for a simple shooter, and I respect the work you put into it. The graphics are nice. The music is fantastic. Mushroom controls well. It does have potential to be better. Your games work fine, but I think you could benefit from some polish. That's something that will come with experience if you continue to educate yourself on game creation. I'll leave some suggestions... You can take them or leave them as you wish:
I got this odd glitch: I went into the shop to "pause" the game, then got up to do something. It did stop the clock, but when I came back and left the shop, the screen was FILLED over fifty monsters. It was as if they just kept spawning while I was in the store, even though the timer had stopped. Even stranger, when I clicked "back to menu" and "play" to start a new game, there were still dozens of enemies all over the screen, and the game would end instantly. I had to reload the page to fix the problem. Maybe a pause button would have prevented this?
One thing that stood out to me is that there's no visual clue that Mushroom has been hit. I didn't realize I had been damaged until I happened to glance at the HP stat. If you don't want to add a "knock back" graphic, have Mushroom flash a few time so the player knows he's taking damage.
For that matter, a small "enemy defeated" graphic would be nice, instead of having them just vanish. Maybe they "pop" or "disintegrate" or whatever. It can be simple. But watching a good "defeated" animation with a nice sound effect can be the difference between a satisfied player and an unsatisfied player.
Can you make the stats (health, ammo, coins, time) larger or darker? Found them difficult to see. You don't need that portion of the screen for anything else, so why not make them bigger?
You might want to consider the shop. Other players might not have an issue, but considering that the action is controlled with arrows + space bar, it seemed awkward the I had to reach over for the mouse if I wanted to enter the shop. What I would suggest is using the same mechanic that other programmers use, where pressing "up" or "down" key while standing in front of the shop will cause you to enter. "Down" arrow would be perfect, because Mushroom doesn't duck, and that key is not used anywhere else in the game. You can keep the mouse controls for inside the shop. That's less hectic.
The enemies look nice, but most players would like to see some variation as the game goes on. Maybe add a few more monsters.
As Sumthin'Fishy says, the player can end up in a situation where they have no ammo and no money. Is this something you did intentionally to force the player to manage resources? Because if contact with Mushroom kills enemies (even at the expense of health), we should still get coins for beating them. That's a pretty good penalty for letting the ammo run out and not managing money better. But maybe make sure the player gets enough money this way to afford to shop.
Also, consider power-ups. The increase in damage seems pointless, because monsters only take one or two hits anyway (and there doesn't seem to be a difference once you buy it.) Maybe some other ideas? Maybe Mushroom's bullets only travel halfway at the beginning, and you need to buy a power-up to make them travel farther? Maybe one power-up allows his bullets to move faster, or to pass through an enemy and kill two at once?
Think about all the possibilities. You can do anything!