Okay, so Shelly graduated top of her class in law school and aced the Bar. The reason she doesn't practise law is because she's afraid of judges. Fair enough - judge-phobia is reasonble in the Scary Go Verse. This doesn't alter the fact that there are all kinds of things a trained lawyer can do without ever having to appear in court, and almost all of them would net higher pay and more respect than working as the assistant of a bumbling small-town mayor. Shelly may be a bit scatter-brained, but surely somewhere along the line, someone like her parents or her former tutor would point out her alternatives.
She may not get a lot of monetary benefits, but she manages to stop anything from destroying the world by working for the ruler of the town where all the evil scary things seem to come from.
Since these days in the comic she's working as a waitress at Hugo's, this is a very reasonable question. My best epileptic tree on this is that, since she didn't realize her judge-phobia until she was in court, her hysterical reaction to the judge has branded her an object of ridicule in Lawyer circles. "Oh, you are Judge-phobic Shelly? They tell jokes about you at the Lawyer club!"