Especially considering her frilly magical girl outfit. We rarely see it in the anime anyway, but it just screams stereotypical. I kind of wish they gave her a better outfit...

I think how stereotypical Madoka's (initial) magical girl outfit is kind of fits with the character, honestly. She's unique among the magical girls in the series in that she seems to want to be a magical girl. She sees the role as desirable in itself, rather than as a means to obtain a wish. At one point, she's so willing to look for an excuse to do it that Mami telling her to wish for a cake is enough. Only witnessing Mami's death, followed by her body being consumed by a giant hellspawn caterpillar, makes her reconsider. There are probably some ulterior motives to her desire to fill that role (Kyubey's suggestion that she wants power for its own sake seems to flip a switch for her, and he makes that his main selling point from there on out), but ultimately, it's not tangential to her goal whatever it might be.

The idea of being a magical girl seems to be a thing In-Universe. It's an odd phrase for Kyubey to use so insistently if no one knew what it meant beforehand, like "superhero" in the West. So, if Madoka actively wants to be a magical girl...well, magical girl costumes seem to be based off of the candidate's conscious or subconscious desires. Her costume would look like what a younger adolescent would think of, when thinking of a generic magical girl.

Like you pointed out, she almost never actually wears it in the show. That's because the more mature version of her that we see, the one that has been influenced on some level by close to a hundred repetitions of a sequence of events that leads to her either dying painfully or falling into a living Hell, recognizes that whatever she wants to obtain by becoming a magical girl won't give her joy in the long run.