They're definitely a bit loose! There's some slack in the etch-a-sketch mechanism too, which is why the tops and bottoms of the curve look a bit flattened. When the motors change direction, the dials must move through a nonzero angle before the mechanism catches and starts moving the plotter. Software can help a little by moving the motor quickly through the dead region.

i built a much nicer version once and it is QUITE difficult to drive the etch-a-sketch rigidly, as there isn't much mechanical support inside the machine. i ended up removing the knobs, adding steppers directly opposite to the shafts, and using a homemade bronze coupler. it worked very nicely but was ugly.