Is it acceptable to use decoupling capacitor ground pad as ground for oscilloscope probe?
In general you want to minimize the loop area when probing fast signals. So, as a rule of thumb, you should select the ground connection that minimizes the loop area.

Now this is only in general. There may be good reasons to use the capacitor ground. This is due to the resonances in the ground plane. Your ground plane will not be zero volts everywhere for all frequencies. It will look something like this:



source

This shows the voltage of the ground plane at a specific frequency. What’s worse is that this can change dynamically depending on the power consumption of the ICs. If you select a ground reference near a resonant mode, high frequency noise can enter your probe, due to the fact that the ground plane reference will be oscillating at the resonant frequency.

The thing about decoupling capacitors is that they suppress the resonances in the power planes. In fact this is how you prevent unwanted resonant modes near your frequency of operation. However this all depends on the geometry of the planes, the value of the capacitor (the smaller the better), power consumption of the ICs, frequency of the ICs etc.

So it all depends on your specific situation. As I said, try to minimize the loop area as a general first approach.