Yeah, but it's a narrow genre so it's not as if tons of movies get made about the subject matter (heck, I think vampire and ghost movies are more common).

Given the number of movies of this type that are out there, I'm not sure that the rating are that bad for zombie movies.

It has had it's share of lemons, but any genre experiences that.

Overall, I think zombie movies have improved somewhat over time (classics non-withstanding).

It has, watch the recently released The Dead, it's fucking brilliant. All those tools trying to perfect the cheesy "zombie love story" should be shamed just by watching it.