edit - This is not a complaint about viability or complexity of materia, or lack thereof, but a complaint about materia in the inventory and mutamix's usability.
Materia should have its own dedicated inventory section.
Also, the game should remember which materia we just gave to Mutamix, so we don't need to reselect the same 5 repeatedly.
Materia takes up so much inventory space and the only "useful" method we have to get rid of it takes ages for any human to use.
It's not even practical to vendor materia anymore since the price was massively reduced.
Alternatively, all materia grades except the two highest grades could be removed, and loose materia could be made dead on a new expansion release.