Connecting parallel to Li-po
It's important under circumstances like this (where there may be several amps flowing through the 0 volts feed), to star-point your 0 volts at the motor controller: -



This will be the first step to minimize data corruptions between arduino and driver controller. That star-point (the big green blob) is "officially" your 0 volts reference point.

But, of course, there will be currents flowing through the 0 volt connection to your arduino that could create small volt drops down its 0 volt feed. Those small volt drops will superimpose themselves on the driver control signals BUT the level will be small and normally tolerable.

If in the process of possible expansion of your design, sensors are attached that might bypass the star point system, you have to avoid this. If the sensors are all teed-off from the arduino then this is unlikely.