Use high-speed optocoupler with slow input signal
Yes, you can certainly use a fast optocoupler for a slow signal. 

One issue is that most 'digital' optocouplers do not have any hysteresis, so a small amount of noise on top of a slow-changing signal could result in a transition that is not clean. In some cases that could be an issue. Say you are counting the zero crossings for a real-time clock. 

Here is what the typical transfer function looks like of a 6N137 (Toshiba). Normally you'd want to run the LED at a nominal current well in excess of that, perhaps 5 to 7.5mA, to account for temperature, aging and worst-case unit-to-unit variations. 



To expand on your example a bit, at 50Hz 240VAC, the mains voltage crosses the zero at about 100,000 volts/second. (100\$\pi\sqrt{2}\times240\$), so it's not moving all that slowly. If you want to detect the zero crossing at, say, 10V +/-20% you have to detect the voltage with an error of 20usec or less. That's assuming no voltage error (say you are using a comparator on the mains side to provide a precise transition voltage). If you allow a volt for voltage error then you must not add more than 10usec of timing error. 

If we use the 7.5mA example, the LED must have 7.5mA nominally at 10V, which would represent perhaps 250mA peak if you were just using (say) a resistor in series with a Zener- which would be a destructive value. So some circuitry is required to limit the current and provide a snappy switching action (minimizing variations with temperature etc.) Some hysteresis and pre-filtering can prevent noise from causing messy transitions. 

If all you are interesting in is counting cycles you may be able to get away with a resistor and diodes, plus some digital filtering with a micro.