I know, why did Derrick target him? Because he was unkempt compared to the other guys? Maybe he has some of that military/police grooming thing going, and equates Donny's look to when he had to go undercover and not be supergroomed among the bad guys? I mean they alternated between thinking he was a professor and special forces...just weird.
And actually surprisingly few messy, unkempt professors---I mean relatively few are into shaving off all their body hair, but very few would turn heads on the street in the nicer parts of town in a big town or city the way Donny's current look would. Those who have longer beards have the Santa Claus look... I assume those with the kind of facial hair that Donny has (which frankly reminds me of hair from another part of the body) keep theirs trimmed short or shaven. Even in the late 60's and 70's they went more for the pornstache type of facial hair than what Donny has.