Hera is, as she says, the "goddess of family" (55.25). Family is usually thought of as a good thing, signifying love and togetherness. And Hera does the love and togetherness thing—she is trying to reunite Roman and Greek demigods, so they can all be happy and fight giants and have picnics. She's like the grandma who wants the biggest family reunion ever.
But family also has downsides. Family means nutty, exasperating, and sometimes dangerous relatives—like Leo's Tía Callida who lets him play with knives (and is really Hera in disguise). It means dealing with petty jealousies and nastiness. Hera and Thalia couldn't possibly hate each other quite so much if they weren't part of the same family (Hera is Zeus's wife, and Thalia is his daughter… it's a recipe for pretty epic feuding). Family in Greek myth is always a very ambivalent thing (check out our thoughts on family in the "Themes" section), so it makes sense that Hera, as goddess of family, is not just sweetness and light, but also hectoring and playing with knives.