Why doesn't Yuuko care about shamelessly revealing her astral body, but is deeply embarrassed by her corpse? Two reasons:
One, Niiya is likely the first person to appear in years (or even at all) who can see Yuuko as she actually appeared in life. She's probably used to not caring how her astral body would look to someone who could see it, because there wasn't anyone who actually could see it.
Two: it's not her actual body, but just her appearance. She doesn't age, and we've had no reason to believe that she can get hurt. It's not a physical body and doesn't require the maintenance of a real body, so she doesn't treat it as "sacred" or "private." But her physical body is and was that, so she feels that it should be treated with such respect.
Why can't Yukariko see the normal Yuuko, but she can see shadow Yuuko? It's because of her own guilt for her part in Yuuko's death. It's established in earlier chapters that Yuuko's appearance to the individual is affected by that person's expectations, and because Yukariko feels regret and wants Yuuko to kill her, she can only see the part of Yuuko that reflects this desire. Seeing the happy-go-lucky, amnesiac Yuuko would conflict with her expectations by making Yukariko consider that Yuuko isn't actually dragging people into death with her and has a part of her that is somewhat at peace in death, so she can't see that part.