I think I'm late for the party, but...
Is there a lot on there, or just a smudge? Mascara can be pretty gloppy, and is very highly pigmented. Do not rub this stain. Do not use soap and water.
I would use a solvent-based spot cleaner, and dab with a paper towel, or something very absorbent. The only issue I see here is that the sun visor is a vertical surface, so you're really going to have to make sure the solution is soaking in somehow.
Next time, do it in the bathroom like the rest of us, Wet One.
