I don't see any terrible imbalance in cut-n-pasting the Toughness powers, changing 'physical' to 'mental' wherever it appears, then calling the result 'Mental Toughness'. On the whole, it would be weaker than physical Toughness (because mental attacks are less common than physical ones), but when you needed it, it would be nice to have. Note that I would rule that neither the armor nor the extra stress boxes granted could be used to aid in spellcasting, as doing so would be imbalancing (YMMV, but note the sidebar on YS250).
As an alternative to this you could just have a stunt/power that granted +X to Discipline when used to resist a mental attack, which is a bit better than having the same amount of armor.