No, Hermione was not at all harsh towards Marietta Edgecombe. Firstly, Hermione had placed a jinx not on Marietta specifically, but on anyone who would betray the Dumbledore's Army. Secondly, even if the jinx was specifically meant for Marietta, Hermione's actions are more than justified. The Dumbledore's Army was a serious pursuit among the students of Hogwarts to learn defensive spells at a time the Ministry was deliberately being careless. Anyone who threatened the safety of its members after willingly admitting to be loyal deserved this punishment. However, since Marietta was a teenager, once again, it would be unfair to judge her for this beyond reason.