My kittens were just born and they are stacking on top of each other while nursing. I don't know if that's bad or ok?
It is very common to see kittens and puppies bunch together and cuddle for warmth and comfort. You should not force them to seperate to avoid stress and risk them cooling down too much.
If the mother lays down to nurse her kittens, you see that her teats are arranged in two rows. If every kitten nurses, there has to be one laying down to suckle on the lower teat and another on top to suckle on the top teat. If this kind of stacking were dangerous for the kittens, nature would have found a better solution to arrange the teats. As it is, it's perfectly normal and not dangerous.