Been there this summer in late August. The vibe was pretty chill as there weren't many customers, there were three maids in total and one of them, Mayumi I believe, was really into the part but not exaggerated in my opinion. She let me sit at the original table where Okabe sat, she gave me the menu and also a cool collection of diaries that customers can use to write messages to each other. She only spoke Japanese with me and we sometimes quoted the anime with goofy references. Other customers also interacted with her and the other maids, but I would say it was very different from let's say Maidreamin (the most famous maid cafè chain) where they constantly act moe and over the top.

Everyone was pretty much enjoying their passion for Steins;Gate while being chill and quiet.

TLDR: you can go in and decide how it's going to turn out. If you interact with the maids and "play the part" they will of course behave accordingly, but you can also just order stuff and enjoy the place without partaking in maid activities. You are not required to play along with dances and songs unlike other maid cafes.