A rude and intolerant woman, Aunt Marge is like some sort of nightmare relative come to life. You know the one – the cheek pinchers, the ones who say really inappropriate things, the ones who passive-aggressively insult you. Marge doesn't mess around with the passive part there, though; she's just downright aggressive.
Aunt Marge wants Harry under her eye at all times, so that she can boom out suggestions for his improvement. She delighted in comparing Harry with Dudley, and takes great pleasure in buying Dudley expensive presents while glaring at Harry, as though daring him to ask why he hasn't got a present too. (2.2.1)
Marge is fat and loud and obnoxious. She's judgmental, she's too fond of alcohol, and she has it in for Harry. The insults she tosses out at him, especially regarding his parents, are truly awful, and we kind of have to agree with Harry that "she got what she deserved" when Harry inflated her even further when he lost his temper.