I though about that rule myself and I don´t really want it that easy to hit an unarmored character critically that easily.
So my rule will be: If you attack someone who is paralyzed in non-heavy armor, you may chose to try for a critical hit or not. Only if you don´t make a critical attempt you attack vs Dex=0. If you try to make a critical hit, you attack vs dex=10 (-5 penalty). The reasoning behind that is that it is not that easy to aim for the most vulnerable parts.
I rule stunned to just drop to dex=10. Down to 0 may be realistic, but advantage and dex=0 may be a bit too good.