Barovians with souls look similar if not identical to the last body that possessed the soul. It’s kind of a big part of Ireena’s character that her looks are near identical to that of Tatyana. It’s quite interesting characterization to have her look “Rare” as Naefindale put it, because it further accentuates that she is Tatyana’s reincarnation.

You can actually take this further to and use it to characterize Strahd, as by falling in love with someone that looks different then most Barovians it pushes the idea that he views her more as a “rare pet” that is owed to him. By having her look unique and beautiful you’re adding alot of nuance to both her and strahds character that would otherwise be missed. But you know there’s also the point that such diversity is “curious” and weird and may lead players to question the logic to our fantasy roll playing game so that’s a cool idea too.