First of all let me start by saying sparky is the most high risk high reward card in the game after three musketeers, at lower arenas namely frozen peak and royal arena it nukes people who don't know how to counter it but once it gets to legendary people have more knowledge of how to counter it.
Okay let me say that I agree with everything you say, especially that Sparky doesn't need a buff (I have Sparky too, so I'm not moaning about it) but this is a wrong, very wrong way to solve the fact. It remember me of the Arena 2 Witch question: you can't buff Witch too much, since she'll be a massive stomp-stone in lower arenas (just like Prince was in the first 2 months of the game, ya remember?).
Now, that said, Sparky's issue is that she's a legendary. A legendary card means that few user have her, and not everyone MUST play her... but ofc, in lower arenas (from 6 to 8) Sparky can be abused since you have not ideas of how you can counter her, and this is her issue.
Tl;dr: Sparky's issue imo is that she's a legendary card, and games where she's involved are always a "stomp or get stomped", but more stomp in lower arena rather than highers.