The way Mary sees it, she’s neither short enough to have speed and agility on her side nor tall enough to have that extra strength behind her punches. She’s taller than average, sure, but that means shit when there’s so many people out there who’ve got her beat when it comes to height. And yes, she knows that there’ll always be someone taller than you, but she just wants to be a Tall Person, okay?
However, what she lacks in height in order to look properly intimidating, she tries to make up for with clothes and accessories. Leather jackets, combat boots, thick bracelets, chokers, fishnet tights, i.e. the ‘it’s very important to me that you know I’m dangerous but also hot’ vibe.
(On a less joking note, for Mary height is directly related to the body as a whole and the body itself becomes a means of empowerment, not just expression. It's not just about challenging gender expectations or embracing sexuality and sensuality or even rebelling against said expectations and norms (though we all know Mary loves a good rebellion) but it's also very much about making a statement. Beyond the surface level of 'I wear chains therefore I'm metal', Muggle clothes are also a statement in and of themselves. Even being as involved as Mary is with the Wix World, she dresses very much like a Muggle which of course leads to the very interesting question of how she perceives herself i.e. more Muggle or witch… which is a whole different character development question so I'll end my ramble here. Basically while Mary is most definitely not particularly concerned with or even interested in clothes, they’re a very important element of how she presents and even views herself. I’m pretty sure she herself doesn’t realise that, though, and instead just sees dressing up as another mindless chore.)
Star Wars reference: C-3PO