Really, avoiding a spell is no more powergaming than being immune to an element, or being able to dodge a crossbow bolt or what have you.
I am not certain that this is the same thing, though. Let me take you through my thinking.

We're not talking about avoiding an offensive spell by dodging, we're talking about destroying any kind of magic. This includes magic that would have effects other than offense; buffs, barriers and suchlike that a player can't actively "avoid" because they aren't using them in that way, and this then forces not only magic aimed at you to be negated but magic cast by them on themselves. The effect of what you're talking about is essentially, without prejudice, instantly destroying those magical effects, which does leave a player without the right of reply.

Shinsou uses [x] buff, Maddy destroys the buff using "No". <---That's where I'm going here.

If the intention of the spell is to negate offensive / projectile / area of effect spells then that's a different story and you'll need to amend the description. As it reads now though, it's an all-encompassing magic killer of all types.

I hope that makes sense!