There isn't a true transmog option in Baldur's Gate 3 so players may be a little bit disappointed to find that they can't equip heavy armor and have it look like a nightgown. This might feel like an oversight but the concept is a tad immersion-breaking and letting players break their own immersion isn't always a wise idea.
Baldur's Gate 3: How To Change Clothes (& Hide Helmets)
Part of the roleplaying experience in Baldur's Gate 3 is embracing a style that suits the character and players do have some say in that matter.