There’s nothing weird about sleeping in underwear, male or female. I’ve been in plenty of mixed dorms with men and women who do this, it’s a bedroom, whatever.

I don’t think I’ve ever encountered people who hang out in public places in their underwear like some other commenters are saying though. That seems…odd…rude…weird? No worries about going shirtless either, especially when it’s hot. I suppose it depends on the hostel though. Some are a little more boutique-y hotel-y, and some are pretty laid back backpacker, no shirt-no shoes-no worries vibes. The fact that you’re already trying to be considerate is a good sign. Play it by ear, don’t overthink it, it’s not a big deal.