Everyone has different definitions of "wrong". Usually, most people would agree that something is wrong if it harms someone. Showing cleavage in most cases would not be considered wrong then. Whether this person is sick or not doesn't really seem to make a difference in my opinion. So personally, I would say that it doesn't really matter and that it isn't wrong.
When I am sick, the last thing I am thinking about is intentionally showing cleavage. Therefore it seems a little strange to me that someone would do that but I guess it depends on how sick the woman is.
I have to agree that it doesn't really make a difference whether the woman is sick or not regarding showing cleavage. I personally believe there is a time and place for being a little more daring. For example, I would be careful not to show too much at work or in a public place. Yet on an evening out with my friends, I might feel that a little cleavage would be perfectly fine.
I'm not sure if you are asking for yourself, or if you have been offended by a woman revealing too much cleavage when she is ill, but maybe this lady is interested in a certain male and thinks that he may feel sorry for her when she is sick and vulnerable, and therefore decides to go for the 'sick but sexy' approach.
It may be that she feels so terrible that she wants to remind herself (and others) that she is still a woman and by showing a little cleavage she achieves that. I don't personally see that as a problem.