Children with ASD do indeed show overall worse performance on a handwriting task than do age- and intelligence-matched controls. More specifically, children with ASD show worse quality of forming letters but do not show differences in their ability to correctly size, align, and space their letters. Within the ASD group, motor skills were significantly predictive of handwriting performance, whereas age, gender, IQ, and visuospatial abilities were not.This is probably useful. Of all the possible causes for the general fact of autists having bad handwriting, they have narrowed it down to the fact that they have a hard time doing little stuff with their hands.
I wonder if it is dose related, i.e., if handwriting (or rather fine motor control) gets progressively worse as one moves through Aspergers to autism to dysfunctionality. My guess is not, or engineers would produce much sloppier drawings than they are noted for.