Yes, use a stereo split cable!
This is a design issue, a mono jack shorts (overloads) the right output channel, and this causes distortion on the left (mono) output.
Also, do not use a straight stereo cable into a balanced input of a mixer, since the balanced input will "see" the difference between left and right. If the output signal from Axoloti is mono, the output is mainly noise (if you crank the gain) since left and right are equal...
As an alternative, you can plug a mono jack into the stereo output, but plug it only two clicks deep. That will connect to the right output channel.