SON OF A BITCH MUST PAY
Well... you still get to attack the other player with the top Super-Villain attack and buy the S-V. Compare that to not being able to buy the S-V if you discard a card with Time Travel and I think that is a pretty decent advantage.
Ah yes I had totally forgotten about how you couldn't buy a card in the same turn that you'd used Time Travel. I guess at least that does differentiate using J'onn J'onzz to activate their Attack rather than discarding a punch to do it
The actual scenario didn't happen in the game I just finished because J'onn J'onzz was destroyed from the lineup before anybody had a chance to buy him. However his appearance in the lineup triggered a short discussion about how his ability would work in this context.
I would agree that using him to play the top card of the S-V stack from this deck would not count as owning the card for it's extra + power (if it has it). Still I'm not sure in this instance that he's worth the 6-point cost, but that's a discussion for elsewhere I think.