Interpreting some functions of an optoisolator from its datasheet
The capacitance stated (18 pF) is the typical capacitance of the diode when it has zero bias (VR = 0 volts). That is the capacitance between the diode terminals.


  Is this the capacitance between the input and the output of the
  optoisolator?


No, that capacitance will be around 1 pF or less. If you look at the equivalent data sheet for the Fairchild device it states a typical capacitance of 0.4 pF and a max value of 0.6 pF. The Fairchild DS also states that the isolation resistance is a minimum of \$10^{11}\$ ohms.


  Is there and hint in the datasheet regarding the maximum CM voltage
  interference it can handle at its inputs?


Given that the diode is "floating" you can make a reasonable assumption that it may be imbalanced to earth with 1 pF on one lead and 0 pF on the other - this I would estimate to be a worst case scenario. External diode drive circuits (even the most simplest) would dominate this capacitance by a factor of ten or so. Basically nobody will quote such a value for just the opto/diode because of this. In other words, the external circuitry attached to the diode will swamp the diode's own capacitance and be the limiting factor that defines the common-mode capabilities.


  Is it possible to say what frequency of CM interference would this
  optoisolator pass through regarding this capacitance?


It has a input/output capacitance of less than 1 pF and it is highly likely that any circuit board it is attached to will increase this by a factor of 2 to 10. It's a capacitor and all frequencies will pass through a capacitor but higher frequencies are of course going to produce more current.