I also thought it was a little uncool that Kyouko assumed that because they're both girls it was okay just come into the dressing room with Yuuri without asking.
I have to say, this is a trope I picked up on a lot in yuri work. Where the LI (generally naively) bullies her way into the changing room with the MC (who's usually aware of their crush at that point), especially in HS aged stories. I'm curious if this lack of boundaries is fully fictional, or is something kinda common in Japan. I never dealt with that kind of thing in HS, but instead in college, but I was a college athlete and just assumed the lack of boundaries was more an athlete thing.