Ok, there's some stuff that you seem confused about here.
Firstly she wasn't sentenced for insulting him, she was sentenced for threats of violence against him, specifically she wrote something like "you won't be able to go anywhere without having your face kicked in" and leaked his personal information.
Secondly she was in jail for 2 days, this was Freizeitarrest which is legally distinct from an actual prison sentence, with the aim of showing someone how unpleasant prison can be. You're not in a normal cell, don't have contact with regular prisoners, it doesn't leave a police record, doesn't have the same probation or anything.
Finally he wasn't sentenced to jail because he was a minor. He did get sentenced to 60 hours of labor with the proceeds going to her. I don't know whether 60 hours of unpaid (presumably) unpleasant labor is harsher than the very short "jail" sentence she got, personally I would say so but it probably depends on the person.
Personally I would agree that his sentence wasn't harsh enough, but German law doesn't really like giving minors harsh sentences. I think her punishment for the threats of violence were completely reasonable.