I have played about halfway through the game so far, and I have no idea how to unlock any of the "Freaking Creepy Mutations" for the zombies. Since you kind of need them for the later stages, one might think that you, as the game developer, might have given the player some indication as to how to do that. Maybe, I dunno, drop a hint or two during the game? Or, hell, since they're outright required for progression, maybe you should allow the player to unlock them after gaining so many stars, or to find them in a level if you look really hard.
But, you don't. It's up to the player to find the missing mechanics. And that's not really good game design.
It would be one thing if they were just secret unlockables, and you could just as easily get through the game with the standard zombie, but you can't.
You can't get to survivors behind barricades without the strong zombie, and I have heard there is a flying zombie, though I have no clue what that's for, since I'm only about halfway through the game, and haven't seen anything that says to me "Y'know, the ability to fly might help me get that guy over there".
Beyond that, this game is not terribly engaging. I understand that you made it in a month or so, and it lacks a lot of polish. And don't get me wrong here, I'm not saying it's bad, per se. I'm saying that the actual game is very shallow, and fails to maintain my interest for more than a few minutes. With your decision to hide away important game mechanics from the player, you've basically ensured that I'm not really going to put forth the effort to keep looking, or playing.
And on top of that, it is dreadfully slow-paced. I get that you're playing a zombie, and they're portrayed to be a tad slow, but really, now. You couldn't speed the character's shamble up just a tad? It's like the guy's wading through hip-deep quicksand! On crutches!
Anyway, I hope that there's no hard feelings here. I need you to understand that my intention with this review is not to attack you, or trash your work. I hope this review has helped you, and you can have a good one.