We're not surprised to see a grumpy Hera in the story of Io. After all, in almost every myth, the queen of the gods is not a happy camper. Mainly because her hubby Zeus is always sneaking off to sleep with some lovely nymph like Io (or whatever minor goddess or hot young mortal woman he can find). Usually, Hera comes off as a total villainess, whether or not it's her fault.
Hera is pretty rough on the heifer-formerly-know-as-Io. For starters, she has the cow-nymph imprisoned by her henchman, Argus. Then, once Io is free, Hera sics a gadfly on her no matter where she goes. In some versions of the story, Hera even sends the vicious Furies after Io, which is way way worse than a gadfly. (Want to know why? Check out their files.)
So yes, Hera does come off as kind of mean, but we can still see where she's coming from, right? Her husband stepped out with somebody else, after all. But why does Hera take it all out on Io and not punish Zeus for his infidelity?