MIM ratio in electronics?
MIM ratio, without context, is likely the area of the capacitor that is required for the mechanical step of polishing the MIM1 metal before the oxide deposition and MIM2.  Due to the fact that this step is often mechanical, you need a minimum size in some dimension to be sure that the tool doesn't eat into the other metal.
This should never actually be an issue as you'll catch it with Calibre during wafer assembly.