They called her Magda the Fat. They could have call her Magda the Nice, had she looked a little more amiable, but she always had had a resting-bitch face. And she never had a reason to smile (she made a point of never let her mouth twitch when the Joker told a joke – it wouldn't do to laugh at some psychopath's sense of humour) so she also wore a scowl.
She wasn't there to look pleasant though. She brought Arkham's residents their dinner, cleaned their cells when they were at their doctor's appointment and she put up with the doctorate snobs that thought they could treat her like a nobody because she earned in a year the money they made in a month. It wasn't the most glamorous job, but someone had to do it, and Magda was willing to do it.
It didn't mean she liked the way they called her. The doctors, the inmates, the guards and even the other elderlies, they all snickered behind her back.
The one advantage of being overlooked by everyone was that while they mocked her, they didn't actively hate her. It was very useful when there was a breakout. Not only did she look harmless (even though she had some experience in brawling) but no one wanted to take revenge on Magda the Fat because they were mistreated by her. There was always that one rookie guard that acted tough and slapped a little too much the inmates around. They'd brag on how they'd beaten up some high-profile criminal, only to be offed the next week when said criminal escaped.
Magda wasn't the nicest, or the prettiest, but she was competent and she knew that the best way to keep her job, as shitty as it was, and her life, was that her reputation shouldn't rise above a stupid moniker for them to mock her.
Magda is inspired by the nurse who appears in the episode "Fear of Victory". She's seen a brief instant as the one in charge to bring the Scarecrow his meal.