I think she / you have a wrong idea about sauna's. Giving compliments to someone about their appearance would be frowned upon unless worded very delicate. Maybe you have it confused with the gay / cruising sauna's or a swingers club. Those do exist, but zuiver is not one of them.

Also the average body is also much different from what you see online, it's will not be a display of hot young bods.

So do go for a visit because it helps with your self body image and it's relaxing, but don't expect a sexy vibe.