It probably isn't that important on low end or medium quality ukes. What follows is pure theory based on some science classes I had a solid C+ in 30some years ago... So, it could be absolute b.s.
Your nut keeps the strings off the fret board. That's really all it does... so a bone nut isn't going to net any real bonus. Vibrations produced from that end have to travel the length of the neck to create sound in the ukes body. Since the neck is a solid chunk of wood, this is not a very efficient way to create sound, which makes the material of the nut more or less irrelevant.
The saddle transmits vibrations through the bridge and into the body of the uke. That block of wood probably retards the vibration enough that it really wouldn't matter, unless the wooden part of the bridge were of a high quality, dense wood. If it's true that the harder the saddle is the more effectively vibration would be transmitted through the wooden bridge, then brass would probably be an even better material for a saddle than bone. Most hardware stores sell brass bar stock small enough to make a bridge out of with a little sanding and polishing. Unfortunately, even going through brass, the vibration of the strings would still be retarded by the block of wood underneath the saddle...