True, Snow's dad didn't come off looking very well, but Regina does have a pretty high body count, both in Storybrooke and the Enchanted Forest. So, while the show is clearly trying to get us to feel sorry for her these days, and Lana Parilla is doing a good job of selling her more vulnerable side, we ought to remember that, as far as the Storybrooke crowd is concerned, she's not just this icy mayor who is trying to turn over a new leaf; she's the bloodthirsty Evil Queen who terrorized them, cursed them, and ruined countless lives. So it's not like they can be expected to forget about all the murders and curses and diabolical schemes just because she brought a lasagna to a potluck!
Also on her rap sheet: she stole Hansel and Gretel from their father, apparently sent quite a few small children to their doom in the Candy House before H&G succeeded in defeating the Blind Witch, caused Charming and Snow to miss out on raising their daughter, separated Geppeto and Pinonchio, betrayed the Mad Hatter a couple of times, and basically wrecked everybody's Happy Endings for twenty-eight years.
Is it any surprise that the townsfolk aren't exactly warming to her now that they've got their memories back?