Actually it don't make you have 75% of chance on magic criticals, it increases your chance of critical on 75%
Lets create a imaginary mage that have 10% of magic critical rate, the increase of 75% of the augment will be actually 7,5%,(75% of 10) so you will have a 17,5% of magic critical.
Seeing that game buffs like Prophecy of Water (that increases 100% of magic critical rate) and Wild Magic (increases 300% of magic critical rate), wild magic augment gives 25% of what the normal buff (Wild Magic) gives, i think is well balanced in the context of the server.
Yet, if we consider that fighter buffs like Might gives 15% of P.Atk, while Augment Might gives 10% P.atk, so it gives 66.66...% of what the actual buff gives, we should say that it's unbalanced in favour of the fighters.