Trixie's odd in that she's a victim of circumstances, but then she responds to them in exactly the wrong way and makes her situation even worse. In "Boast Busters", the Ursa wrecking town wasn't Trixie's fault. But when Twilight saves the town (and Trixie) from the Ursa, Trixie responds by insisting that she's still better at magic (somehow) and immediately running away. I'm convinced that's the precise moment when Ponyville's general opinion of her flipped from "Cool magician! Great show!" to "Wow, what an asshole"—and the moment Trixie doomed herself to never living down the Ursa incident.You're right that we can't blame Trixie for anything she did while under the Alicorn Amulet's effect. One the other hoof, we can and should blame Trixie for putting on the Amulet in the first place. For wanting to get revenge against Twilight (one of the few ponies who didn't antagonize Trixie at all) and for seeking out dangerous magic artifacts to accomplish this.