The Department of Temporal Investigations is on the case

The immersion? I've been to some and people dress up as like fairy's and elves and all sorts of fantasy shit.

Same. It used to be a lot more of historical reenactment, and a lot of the performances and stuff still reflect that.
But the last few times I've gone it's mainly been a themed craft show with corsets and tunics, and yeah, people with wings, steampunk hats, and Cloud's buster sword, or tails hanging out of their pants. :|
Renaissance times must've been wild, y'all. Lol
- IDK man, the more historical Renaissance fairs still exist, with people a bit more strict about dressing period-appropriately. The ones you're talking about are basically a whole separate thing, it's basically a fantasy-inspired comic-con, and attracts a whole different audience.
  I don't think there's anything wrong with either. I think it's awesome that there's places for people to sell their niche handmade goods while sporting the fashions of 17th-century France or whatever, as well as places where people can dress up like fantasy creatures, tails and all.
  
  Oh sure I agree! The slightly confusing bit is the one here is called "Age of Chivalry - Renaissance Festival", but could just as easily be "Medieval-themed fantasy con and craft show" ...maybe wouldn't be as catchy.
  ...but hey I got some sweet custom dice there ¯(ツ)_/¯
- Imma be honest. I don't get the whole LARP scene. It's weird AF. Happy for people finding something they like but damn.
  
  There's a great book on the LARP scene called "Leaving Mundania" by Lizzie Stark that's a really easy, entertaining read. It helped me really empathize with the hobby and the people behind it.
  She kinda had the same perspective but approached it with curiosity to figure out what the fuss was about.
  (Up until that last chapter on "Nordic LARP"... it's been ages since I read it but that stuff got weeeird.)
  It's one of those things that would sound like a ton of fun to me, going all in on pretending to be some fantasy character and everybody else is in on the bit, but there's some combat rules to prevent "invincible OC syndrome" so you're actually moving playing pretend instead of plopped in a chair for 6 hours for a tabletop game.
  In any case, it looks like a good way to escape playing "Bills & Bosses", a game that is near-universally reviled. hahaha
  Unfortunately it's just very geo-dependent and cost intensive I'd think haha.
- It depends on the event and the event holders.
  "Ren Fairs" in the US are more likely to be put on for fun. Go hear a guy in tights play a lute, eat a roast turkey leg, etc. I think convention cosplay culture is starting to leak in.
  If you want to see Americans take period costume seriously, go to a "reenactment." You'll find folks who are very serious about history, many of them are professional historians, archaeologists or museum employees performing for the education of the public. Probably the biggest community is the one around the American Civil War. Go watch the movie Gettysburg. Sure there's the main cast of Hollywood actors, but then there's hundreds and hundreds of extras. They're war reenactors, who showed up with their own personal uniforms and equipment.
- going by your username Im gonna assume youre german. you see all kinds of costumes and clothings at the MPS for example and imho noone is bothered by it
  
  sounds more like „if you wear a medieval looking costume, do it properly“ tbh

A not insignificant number of guests are usually dressed like regular modern day people. Ren faires are for everyone, and most people don't own period costumes. It's usually encouraged for guests to dress up, but it absolutely is not required.

Is everyone dressed up at these fairs?

I don’t see how a few people in trek costumes would crush immersion if plain clothes people don’t either. It’s both or neither, not one or the other.

- It sounds like it's basically the same. My understanding is this type of thing actually started in the US and was exported to Europe, ironically enough. But yeah, many (most) guests are not in costume. I don't see how some guests in Trek costumes would harm anything.

There are some that do themed "Time Traveler" days where guests are encouraged to dress up in steampunk and sci fi costumes. I love going on those days, you always see tons of Trekkies, Whovians, and Jedi running around.

You've clearly never been to one...