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.
Well, since I'm answering this question two hours after the fact, I assume the kittens are out of the "sack" as it is called and washed up by momma and nursing by now. So in this case you've nothing to do but clean up the mess. If a kitten is born and the mother cat will not clean the placenta or sack away from the kitten so it can breathe you must do it for her and clean the kitten gently with a wet cloth. Some mommas just don't get it. Always clear the airway first and if need be breathe into the kittens face softly to give it life. Rub its sides at the same time to stimulate the heart to beat. It only takes a minute for that kitten to die without attention.