In Zombie Piano, tap the keys on the onscreen piano and zombie sounds will emanate from your iPad in an approximation of that tone. The concept is funny to a degree, and while it almost certainly is an attempt to capitalize on the current popularity of zombies in pop culture, it fails to provide enough of an engaging experience to hit the mark.
To be certain, Zombie Piano is not a serious app. You are not supposed to be able to make music with this app and it's not surprising that the zombie sounds barely sound different from each other when hitting different keys onscreen. But there is something inherently limiting about how this app is constructed and ultimately how it works. The keys have a bit of lag between them, for example. Hit one key and it takes a half second or so before you can tap the next key. There are four different types of zombies you can play, but changing between them has the same lag, and there is no option to record or share what you create in this app.
Zombie Piano is a one-note toy that will be funny in certain situations but is almost certainly not something that you open and use more than once. The app is free, of course, and there is no paid upgrade, so this is the full app, but even still, it lacks enough to be worth the download unless you are a big zombie fan.