It sounds to me like you are critiquing a whole genre (and cosplayers) because of a rare edge case.
I think the cosplayers disturb me quite a lot because as a whole, they continually adopt the attire, epithets, mannerisms etc. associated with the oppressors and exploiters of that era, without any apparent recognition that they are doing that, instead seeing it as cutesy, which frankly it is not!
As for edge case, I'm criticising that
specific case (i.e. set in the real world and/or using real historical figures), and maybe that is an edge case, I haven't done the kind of comprehensive survey of the steampunk genre that I'd need to confirm/deny that!
I can think of plenty of historical novels (and TV shows) without SF or fantasy elements that do that.
For sure. But generally those periods aren't also worshipped by people who think the past was "better". Like, we didn't get "Elizabethan Values" or "Restoration Values", did we? We got "Victorian Values". Maybe it's just unfortunate for the poor cosplayers that they like the same aesthetic as a bunch of creeps, but for me that's still concerning.
Just to really boil it down, if nothing else, what I think is genuinely "problematic" is the almost complete lack of awareness among a lot of steampunk aesthetic fans that they're mimicking the attire/epithets/mannerisms etc. of people who were, for the most part, "the bad guys".
It's a bit like plantation weddings in the US, if you're aware of those (especially as many of the same people benefitted from slavery). It's gross and a little "hmmm" to me.
I don't think I've got much more to say about it honestly though.