A counter is a really good idea, after the jam we will see about adding one. Maybe the Lemon Wizard could comment on how many zombies have been killed every X amount of zombies rather than having a counter constantly on the screen.
There is an amazing surprise at 100 kills, and another at 500, but because we didn't have the time to do any balancing I don't think anyone will get that far. We'll be changing a lot with this game after the jam.
Thanks for playing, and thanks for the feedback! :D