I feel very conflicted about the no bra thing.
At the end of the day, they are just nipples and if a lad her age was walking around topless, no one would care that you could see the entirety of his nipples. Mia is wearing a top that covers hers, you can just see them when it's colder because of how our bodies react.
However, I do see it from the other side. We live in a world where a woman's breasts have become massively sexualised rather than being seen as another body part or something used to feed a baby if they so choose. Every single part of the body is sexual to some one but its not expected that we cover every inch of our bodies just incase Andrew over the road finds an ankle attractive. It's not Mias fault she lives in a world where her body is sexualised by people but it is something she should be aware of. If, God forbid, anything happened to her, we know from history that what she was wearing would be blamed by some people.
As for the cat calling. People are disgusting. it doesn't even matter what you're wearing a lot of the time but it does happen more often when people are wearing more revealing clothes. It also depends on where you are. In my home town, i was never cat called but only a few weeks after moving to uni I was, and that was wearing joggers and an oversized hoodie! Mia seems to now be aware of cat-calling after Costa rica and probably knows its inevitable as wrong as that is. But if she is confident enough to wear the clothes she is, i don't personally see anything wrong with it at her age
I have more to say but this is already long enough