Susan originally appears as a haggard and elderly woman in the a field of barley after having committed suicide in spite of being around her forties as seen in the introduction scene. Her unattractive appearance continues for a few chapters until she returns home after having recovered from being murdered by her first encounter with a Parasite, and now appears closer to her age and looks more reasonably attractive after taking a shower to wash up her blood soaked clothes. Why does her physical appearance dramatically change? Simple. In her mind she believes she's ugly, which is a common symptom of people who suffers from depression, especially since she had recently taken her own life. Upon being murdered, only to be resurrected, take her revenge against the first Parasite, and save someone's life as a result, she's now starting to gain a little bit of self-confidence, hence the change.
People suffering from depression also commonly don't take care of themselves and their appearance— it's too much effort for them, and in their mindset they generally don't see the point in looking nice anyway. The fact that Susan's become much more attractive after killing the first Parasite proves that, at the very least, she's begun to crawl back out of the deepest part of her depression.
Early on in the game, she also claims she keeps to herself and doesn't particularly know anyone. The former's true, but it turns out that she's familiar enough with her neighbors to help Mitzi fill in a map of their apartment complex with information she's either known about them for a while or has overheard from their gossip (not to mention she knows Bryan well enough to tailor her telling of the Cat Widow urban legend to put him in the victim's place and know he'd be too proud to bust her for the prank she pulls on him), suggesting she's starting to reopen her mind and awareness to the world outside her head.
Since Joe is shown to have his wife strapped to an electric chair after the sequence in his apartment, it can be assumed that this game takes place before Downfall, which ends with the police breaking into the apartment where the Davises live and finding that Joe has killed everyone in the complex. ...Would that include Susan?
This is actually covered in the 2016 remake. In the remake, Susan intervenes and tries to kill Joe. She strikes him down in all three endings, though in the normal ending, Joe gets back up behind her back as the Axeman and the game fades to black with him cornering Susan. In the Golden Ending, she knocks him out in a slightly more permanent but non-lethal fashion, though instead the newly resurrected Ivy hacks her in the back with an axe and leaves her to bleed to death, though the ending's newspaper clipping reveals that she managed to survive. In the bad ending, she knocks Joe out, and realizing that he's still alive, decides to finish him off by executing him through electrocution before tossing his body on the burning table where Ivy laid. However, this last ending adds a new layer of Fridge Horror when it's shown that she has become more like the Eye of Adam, persuading people into committing suicide and with no clue to what drove her to become that. A number of trophies from the numerous parasites she's slain now adorn her room as well.
The last time Susan returns to life there is an option to refuse to snuff out someone else's life to make it happen. If you take this option, it is revealed that The Queen of Maggots, the embodiment of her depression, had lied to her all this time and all the people she snuffed out before were entirely unnecessary.