Interesting question! It's important to note that when Hero was originally disgraced at the wedding, Leonato was ready to see her dead because of the shame she had supposedly brought upon her family. I mean, his daughter who had always been pure and modest should have been given the benefit of the doubt, but Leonato even says, "Would the two princes lie? Would Claudio lie?" Leonato knew the character of Don John - that he had risen up against his brother and probably wasn't 100% trustworthy - yet he was willing to believe him over Hero.
Then the truth comes out and what does Leonato do for "punishment"? He says, "Hey, you can marry my niece, then, to make up for what you did to Hero." Now I realize that this was all part of the plan to get Hero and Claudio together, but Claudio probably deserved more of a punishment than to simply be given another wife to marry (that was his perception, of course, as we know he's being given Hero).
Personally, I think Leonato is a typical man of his time - he put more faith in the honor of men than in the honor of women. Shakespeare was highlighting the irony of this in this play, particularly with his song, "Sigh no more, ladies," which talks about how "men were deceivers ever." So, from a modern perspective, Leonato's punishment of Claudio wasn't much punishment at all. But I think that is the message Shakespeare was trying to get across to his audiences.
i think that i is a little bit under harsh as Claudio disgraced his only daughter Hero and by making him agree to marry another woman was a bit simple.