I just generally think the crazed eco-terrorist look works much better if she actually looks human.
By coincidence, I recently re-read an old human-looking Ivy story scripted by Neil Gaiman which was, well, average - the art let it down. But there was a particular panel which, taken out of context, could heavily imply that she masturbated to Batman images while in college. This would probably make Batman at least a decade older than her, so it may be a good thing the story was discounted.