That's up for debate as far as I'm concerned.
It affecting cloak would give birth to a very strong combo, but it would require splashing or running a rainbow.
At the same time, it would encourage use of AoE spells, which we see less often.
Since cloak lasts only 2 turns, I wouldn't think it would be too OP, as the chain would break rather easily, so my current standing is why not.
In terms of Earthquake, I would like to see it not get targeted by it. 3 Magnets gone instantly from the stack is quite a lot in terms of protection, but at the same time, it might be too much to have earthquake completely denied by it.
On the other hand, Earthquake is seen as a borderline OP card, and a minor nerf like this might be good for the metagame.
Those are my thoughts, but I will put up a poll.
Umm... quick question. Does having 2 Magnets on the field make every permanent permanently invulnerable? One Magnet protects the other.
No. They stack on top of each other, so only the one pile will be targetable. That way, they don't protect each other (Mentioned in notes)