Your game is frustratingly bad, and I say frustrating because it seems like you tried to make it good but it just turned out bad.
Graphics:
The art is average at best, the backgrounds will always be random clouds shooting past you with blue sky so nothing interesting their. All the character designs are about as generic as it gets, the ninja looks the same as any ninja I've seen in hundreds of other places. The standard zombie is your typical green slightly disfigured zombie for some reason falling out of the sky. You had some unique stuff going on with the exploding zombie strapped with dynamite. The animations were very basic with only 3 different states for the ninja, he's either falling...dying...or slashing and that's it. The power ups are the most obnoxious thing i've ever seen, the entire screen goes through a blender as you blur the crap out of everything, and on top of that oversaturate with color so the game lags and I can't even see a damn thing. The only thing that stood out to me were the gore of zombies getting dismembered was pretty nice looking.
Gameplay:
Now this is where it gets real bad. The gameplay is so repetitive I had to drag my feet to level 10 just because I wanted to see what you had been touting as the game picking up. You'd think common sense would tell you that if I have to play your game for 15 minutes all the way to level 10 just to enjoy it there's something wrong. Even after level 10 the gameplay doesn't really pick up all that much except that you throw in some exploding zombies and I don't know maybe you added more but I got so bored I quit after level 13. The gameplay suffers even more because you hide the score from the user in a score-driven game. So there is no indicator as you're playing of how well you're doing. I don't have a multiplier indicator yet I'm picking up dozens of multiplier power-ups in the game, so it ends up having no meaning at all to me. I don't get to see how my score is affected until the end of the game.
Overall the game would've been average, but with all the poor design decisions it ends up coming up short at its core. You need to understand that people play flash games for free so they have no money invested in your game and will only give it 3 to 5 minutes to stand out to them or will move on to another game. I think you can do better but you need to make better choices about what is fun and what is repetitive.