I think it's a fun looking image, but I agree with @eishiya on the critique -- it really doesn't read like a "zombie" should. I'd say more of a Frankenstein or "Gorillaz" styled character.
Even so, you should consider avoiding so much symmetry in the face (perhaps a drooped eye or eyelid, or sagging skin on one side of the face?) because the "cuts" across the face just doesn't suggest "decay" to me.
To compound this, the guy looks WAY too happy to be in such bad shape (or "dead" as it were) and if you were going for the crazy-eyed look, then you should give him something that makes him look a little more forboding (i.e. fangs, missing teeth, etc.)
Above all, the image doesn't look bad, it's a fun design, and your pixel technique is pretty decent, but there's just not enough going on, he doesn't strike me as a zombie due to many reasons (the green skin should probably be closer to brown or purple because, right now, I see frankenstein with the squared head to top this guy off), and the symmetry is making that "not enough going on" look much worse unfortunately.
I hope that helps give you some insight on where to go from here, if you're interested.