I like the piece as it is, but obviously that wouldn't make for a very useful critique. So, I'll make some suggestions of how it could be different. The difficulty with making a critique of a picture with an intentionally cute and simplified style is that it's hard to tell if the simplified elements are 100% by choice or whether it's more due to skill level and it's something the artist should work to improve.
The characterizations are appropriate and expressive. They don't suffer from the common problem of characters who just stare with no expression. However, the way the eyes are drawn on some of the characters makes it unclear what they are gazing at. For example, the front character (is that Andrea?) looks like she's looking at a rooftop in the distance rather than the zombies in front of her.
The scenery is simple, but of course that's appropriate for a cartoony rendering. But it looks as if the background was very much an afterthought. The overall feel is 2D or 2.5D rather than 3D due to how the characters are situated in the scene. A different camera angle and more engaging scenery would be more interesting, but might not be appropriate for the goal of the cartoony feel. It's a personal choice.
Overall, the piece is well done because it has the desired impact. It's cute, funny, and clever. The PvZ zombies are already goofy as designed, and mixing them up with cartoony versions of The Walking Dead is adorable.