But what is "right", in a genre that's speculative anyway? The reasoning is simple: "I like the look of this corset on the outside, so that's how I'm going to wear it". It's aesthetically-pleasing; that's the thought applied.
How is one able to tell just by looking at another person's ensemble how much "thought" or "planning" went into that outfit's design? Perhaps that costumer's thought process is just different. Besides that, is there a benchmark for exactly how much one should plan out an outfit before they're allowed to wear something they enjoy? Maybe it's not even a costume - maybe they just like the look of jeans, a tank top, and a corset. Are they harming anyone by wearing an outfit that doesn't meet some arbitrary standard for thoughtfulness?
Living history and re-enactment groups have set rules because their purpose is to recreate something that existed in the past. Steampunk is not a genre based on accuracy, and someone should be able to wear as much or as little steampunk as they like - it's a fashion sensibility just as much as anything else, and people mix-and-match fashion all the time. Mostly Lolita with a top hat and goggles? Jeans and a tank with big stompy goth boots? Sure, why not?