I think the randomness is good. I wouldn't make it an activated ability unless it also had the fail chance. I think it's good as is personally. 25% means that it might not copy any spells, but even so this would go into any deck that would run 4+ copies of lightning. Specially for arena submitted decks.
After thought on this card. "IF" people think this is OP as is, then I would suggest it coming into play time bubbled (don't know the correct term) so that it has one turn of PC before it can be used. Again thats only if it's OP.
Love the card, I hope it makes it's way to the top (^_^)b