The outfit seems to be going right up to the line IMO, and I could see an FA asking her to cover her nipples-up area, especially if she tended to "fall out" when seated.
It's one of those one-off things where everyone could have done better - the FA could have handled the cover-up request better; the pax could have done a better job thinking about how the outfit would work on a plane flight. Looks like she got her travel refunded, so it's kind of a nothingburger at this point. <shrugs>
It does bring up a fun thought exercise: If a male arrived with a strapless tight "shirt" on, with 100% nakedness from the top of the head to the top of the nipples...would it make a difference to anyone here?