Toothbrush moustache was common amongst many comedians of the day. Because it doesn't cover the corners of the mouth it doesn't cover up the creases/etc so considered less inhibitive to displaying emotions. It makes the expressive bottom lip appear the more prominent .Charlie Chaplin another famous comic of the day with one (Hitler idolized Chaplin)... Chaplin's was a fake stick on one though. Clowns will only paint one side of the lip darker for same reason (make bottom lip more prominent)... It's more expressive of emotion... The Same psychological idea behind the toothbrush moustache.
