Stargirls' best characteristic is her ability to empathize with anybody.  She cares deeply for the feelings of others regardless of social norms.  It's why she cheers for both teams and that kind of thing.  She expresses the emotions that other people should also be expressing.  So when Archie says that Stargirl "doesn't shed tears, but light," what he is actually pointing out is that Stargirl is enlightening people to the emotions that they should be feeling and expressing.  A lot of people turn a blind eye to somebody else's embarrassment, or worse still, they laugh.  Stargirl would run to help and emote the feelings of the victim.  That's what other people should be doing.  Her tears of light are enlightenment.  Her tears bring knowledge.  Cartoons use this motif all of the time when an actual light bulb is shown next to their head.  Have you ever heard the phrase "could you shed some light on this for me?"  It's another way of asking for something to be explained. Stargirl explains things through her tears.