It seems that Jacob failed Dinah in several ways. He put her outside Shechem where she should have never been. He allowed continuing worship of pagan gods in his home. He was concerned more with his personal honor and image than that of his daughter. Moreover, he left it to his sons to deal with this tragedy rather than taking a leadership role.
Where was Leah in all this? The Bible is silent about her. Should she have known where her thirteen-year-old daughter was? Maybe she did know; perhaps she allowed her to go to Shechem unattended. The pagan festival held there may have been familiar to Leah, and she felt no threat from it or its attendees. An ancient Jewish commentator on the Torah, Rashi, alleged that "the daughter is like the mother," managing to condemn both of them in his short comment. Notice that Leah—the mother of six of the tribes of Israel, part of Christ's genealogy, a woman blessed in so many ways by God—is blamed rather than any of the men.
Are Hamor and Shechem to blame? Certainly, Shechem must take blame, as he raped a child, one in mind if not in body. In some respects, despite being the perpetrator of a dreadful crime, he comes off as somewhat honorable. He is said to have loved her, he appears to be sorry, and he offers to marry her and give the family a dowry.
Hamor's role in this seems to be two-fold. He wants to give his boy his desire, and he wants to accumulate more wealth and power for himself. When he pitches the idea of circumcision to the townsmen in Genesis 34:23, he says "Will not their livestock, their property, and every animal of theirs be ours?" Hamor means "ass" or "donkey," so make of it what you will.
Then there are the sons of Jacob, led by Simeon and Levi, full brothers to Dinah. There is no excusing the crimes that they committed, and they are subsequently punished (see Genesis 49:5-7), a part of the saga that is a story unto itself. They certainly share great blame.
What about Dinah? Was she a hussy? A shameless and promiscuous girl? It is interesting that in this entire chapter we never hear from her. Was the rest of her life ruined? Did she have a child from this rape? Many interesting theories have been suggested about the remainder of her life.
My personal opinion is that she was in the wrong place at the wrong time. However, she put herself in that situation, so she does carry some blame for what happened. She does not deserve all of it, as the older commentaries have it, nor was she some sort of feminist out to break the hold of her male-dominated world, as more modern commentators try to make her.
We cannot know all that went on in this matter, but God has given us a great deal of information about the various players. As with most things in life, there is more to it than what appears at first blush. There are layers to be peeled back and meditated on. When examined from all angles, we can see that none were truly innocent in the Rape of Dinah, which is a lesson for us all.