At least at the Faires I've worked at no one cares.
In fact, I have a second-hand anecdote that one faire staff has a screen accurate combadge inside their doublet. When they see people doing stuff like this, they run over and pull them aside. They would whip out the badge and get in their face. "Does the Prime Directive mean NOTHING to you?! This is an uncontacted pre-warp civilization. You are ruining YEARS of undercover research! Who is your captain? I'm going to have your court martialled if you don't get under cover FAST."
...considering the term is nearly sixty years old i'd say no.
Edit: exposing myself as an olde farte but in fandom circles back in the day there was a sharp line between trekker and trekkie . Oooh you didn't want to be seen as a trekkie, they were the fans that ...well looking back at it, the trekkies were openly obsessed over the series and had all the fun cosplaying and being free about it, the trekkers were the ones that pretended they weren't mad about trek and thus were allowed out into respectable society. Cringe factor i suppose. Heh. But old name habits remain.
Fair enough, though I'm not too far behind ya there. I suppose it's similar to the dichotomy of geek v. nerd, insofar that the distinction is lost on those outside the group (much to our chagrin/cringe). 🖖🏽
It's also an age thing - I mean every fandom has Those Fans, but in general being a fan of something and buying / wearing merch is now nbd. Nerd's gone mainstream.
Back when those terms were coined? Sooo not the case. Like, it could literally get you beaten. Wild times.
I'm similarly minded. A friend and I have talked about doing this for some time, but it's such an old joke I sorta fear it wanders into the same territory as asking the grocery clerk if the item that didn't scan is free.
My favorite, when I was working any last point of contact with customers: "It's only a crime if you get caught. At least, that's what I figured when I wonder why mgmt gets a cut of my pay.", and the deadpan is essential. 🤘🏽
People at ren fairs are usually in pretty good spirits and good humor. Considering there are people dressed as wizards and elves, I think they'd be okay with Star Trek time travelers.
I've seen a story where one of the faire participants had a commbadge hidden in their costume, pulled the Trekkie to one side, showed it to them, then told them off for breaking the Prime Directive...
In my experience, you might annoy the "hardcore" ones, but most people won't be bothered. Hell, people may honestly join in, so long as you aren't causing any problems.
A lot of the Faire folk I have met are just happy people are participating and having a good time, and if you put effort into a costume, even better.
Considering what goes into the more hard-core outfits, they deserve respect for their dedication. And there are lots of outlets for that too, which is great (e.g. SCA).
However, if someone gets their tabbard in a bunch because some sci-fi cosplayers ruined their sense of immersion, at what is arguably a pay-to-enter medieval-themed shopping mall, they may deserve what they get.
A lot of the Faire folk I have met are just happy people are participating and having a good time, and if you put effort into a costume, even better.
These people are the backbone of every Faire. Huzzah!