I feel like that most Zombie Players are UP. I guess thats hard to buff though ._.
you are right that Plants are easier to learn to begin with. Once you got used to the slightly more complicated gameplay both sides are en par.
It appears that most players just don´t have the patience to learn how to play zombies properly.
Trust me; I am a very spiky, tryharding player and ranked 50 in every season so far and I can assure you there are remarkable Zombie players out there that do not feel underpowered AT ALL.