First of all I think some people here don't take into account, that if the nymph uses her ability the field isn't crowded with Malignant Cells the next turn. I don't want to speculate on how long it takes on an average base. But the opponent has quite some turns to play quite some creatures after one of them has been aflatoxined. And remember: An afla'd creature has to die first before the m-cells even *start* filling the field. And then (!) you, the nymph player, still needs a shield (that may get PC'd) when you are facing an army of (unupped) cells.
I suggest removing summoning sickness or making it untargetable for one turn. Then maybe (I'm really unsure about that particular point) the skill cost could be raised again. So in the end you have an expensive afla spell in the first place. But you can cast it several time so it is more efficient. As a rare card the nymph should be somewhat powerful after all.