Mother cats will usually eat the placenta for two reasons. One is to keep the birthing area clean, and the other is that the placenta is high in nutrients and this can help the mother to recover some of what she lost during her pregnancy. If she does not eat the placenta, it is not "bad" but you will need to remove it and throw it away for her. Do not take it away immediately though. Labor can be exhausting, so she may just be resting before she decides to eat it. If it has been there long enough to get dried out, then she will probably not eat it and you should take it away. Make sure mother cat is on a good quality kitten food while she is nursing. Making milk takes a lot of calories and the kitten food will help provide her with the extra nutrition she needs to have good milk for those kittens.
