If you are in undead form (Zombie, Lich, Wraith, Vampire etc.) you automatically get 100% fortification (which stacks with all other types of fortifications), so if you are already wearing a light fortification item (25%) and in undead form you have a total fortification of 125% which will completely save you from enemy critical hits/sneak attacks no matter what level you are.
About the robe as I mentioned earlier, wizards/sorcerers will never be able to reach high enough AC without sacrificing their spell power (read: usefulness) to remain safe. So, if I am not a pale master I would get that +0 robe of moderate fortification than a +5 robe of anything else. However, if I am a pale master then +0 robe of moderate fortification will give me 175% fortification which is just too excessive as the highest I need is 125%. So I will rather equip my wizard with something else.