Before I comment on the card, I have to bring this up:
i think of it as a spell because it melds all creatures together into one, +|+ being the atk|hp basically seals the deal to me.
So having attack/defense with variables instead of having a spell symbol makes it a spell to you? Oh the irony.
Now, about the card itself. I love the interaction with many cards in the game, and I do think it's balanced as is, although +1 cost certainly wouldn't hurt any.
You think it should cost more? So it would be 4
|3
? I think i'm opening a poll for that!
Does Mimic copy passive abilities as well?
Good question! At first i thought that passive abilities shouldn't be copied, because for some reasons i thought that a creature can only have 1, like active.
But now that you are saying it, i think that the more accurate the copy is, the better.
So, yes, i think i should copy passive ability as well. And i'm adding it in OP.