what was jems punishment? did it fit his crime?

Punishment number one was cleaning up his mess and tending the flowers to insure they'd come back healthy.

"I cleaned it up for her and said I was sorry, but I ain’t, and that I’d work on ‘em ever Saturday and try to make ’em grow back out."

Secondly, Jem has to read to Mrs. Dubose for a month.

Jem seemed fascinated by a rose in the carpet. "Atticus," he said, "she wants me to read to her."

"Read to her?"

"Yes sir. She wants me to come every afternoon after school and Saturdays and read to her out loud for two hours. Atticus, do I have to?"

"Certainly."

"But she wants me to do it for a month."

"Then you’ll do it for a month.

All in all, I'd say the punishment fits the crime. He has to care for the things he attempted to destroy, and yes, do something as easy as reading outloud to an old lonely woman. Note, this is something that doesn't last an afternoon...... it something he doesn't want to do....... and he get to do it for a month. I'm pretty sure this tactic reminded him of a lot of things as he got older, including something you've proabably haven't gotten to in the novel yet.