You want more insulation on the bottom than on the top. It is easier to vent topside insulation and sleep more comfortably. That is how we sleep on normal beds and we have developed instincts on how to vent, roll over, tuck in covers, etc, in our sleep.
If you have more insulation on top than on bottom and you get into a situation where you are cold on the bottom you can get into a bad situation. When you are cold because you don't have enough insulation on the bottom, your 1st instinct might be to tuck in your top side insulation (since that is all you can get to) and overheat on the top. That doesn't help and isn't a good situation to be in... actually it can be pretty miserable.
Whenever you are camping and wake up cold, you need to try and determine just where you are cold. Then you need to think just what you can do to remedy that. Sometimes it is just a simply adjustment with your gear and other times you need to do something. Gloves, hats, unused clothes, etc can be used to quickly shore up a cold spot. Small pieces of closed cell foam can do the trick as well. Sometimes it might be wind getting to you and you might need to get up and change how you pitched a tarp or move something to block it. The wind is another reason you want more bottom side insulation than top side because the bottom side is generally more exposed to the wind.