Haha. Yeah I suppose you are not really familiar with the Magical Girl genre (^.^;
As for how "far off the mark" you are... it's a little bit off; but Magical Girl is basically a normal girl that gets magical power -- so in that sense you are not off the mark.
But what people typically think of when they hear Magical Girl, it's something like this:
(Kaname Madoka from Madoka Magica, the Japanese animation whose 12 episode titles became this jam's theme)
This cute Japanese-style (though I suppose a western cartoon "Power Puff Girls" definitely fits this genre) cute girls wearing cute dresses. When they get their magical power, they can transform into their magical girl form where they wear cute outfits. For your game... it's kind of out of the mark in how she wears some sort of magical armour... Usually magical girls wear cute dresses, and more importantly: transforms :3
A quick fix would be if she still wears armour, but instead of physically wearing it, she instead can transform into her armour outfit (but probably no helm; no magical girls cover their faces)
Your game definitely still fits the basic definition; it's just kind of not what most people (or at least most lovers of Japanese entertainments like myself) think about when they hear "Magical Girl" (；´∀｀)