okay. here's my thoughts. hopefully these are coherent.
koala doesn't wears clothes that might show her sun pirates mark, and only shows her sun pirates mark to people whom she is very close to. most of what she wears covers up her back, and she doesn't put herself in positions where people might be able to see it especially when she isn't on base.
and it isn't out of shame, either. koala doesn't seem like the type of person who feels shame about her mark. to me, it boils down to two reasons: one, everytime she sees it there is a lingering reminder, no matter how distant, of the circumstances behind why she got it in the first place (because trauma doesn't just go away like that). and two, and tbh what is to me the main reason, seeing it could raise some questions from people who don't know her as well as say, sabo hack and dragon.
like. she seems to work mostly in gathering intelligence and only seems to take an active fighting role alongside sabo and hack in very specific situations, so she really can't have anything that might draw attention to her. and seeing the mark of the sun pirates would almost certainly raise questions in any nation that she's in, not to mention seeing it on a human would raise more questions. it's probably much easier to just have it covered up than having to explain it over and over again.
hell, i would say that even on baltigo she probably either takes private baths or if there is a communal bath she probably waits till most people are asleep to bathe, because any new recruit who doesn't know her will see it and will almost certainly have questions, even if they don't ask her directly.
i also think this is why i really love when people draw koala with her back exposed and remember her sun pirates mark, because it is such a huge part of her, but because you don't see it much it's so easily forgotten. but it is there. and it is part of her character.
