Tom Riddle in canon was described as a classic charming psychopath, while the Defense Professor seems to be genuinely icy and blind to others' internal states. He even verbalizes this a few times, e.g. "I don't have the knack." So either HPMOR!Riddle was actually not charming but instead vastly more intelligent in a cold, calculating way, or the Monroe/Quirrell persona is supposed to be outwardly cold and obtuse while still secretly possessing insight, or this fragment of Riddle's soul has lost whatever insight or ability that formerly made him charming, or something I haven't thought of.