She is evidently attracted to men with tendendies towards random acts of violence; falling for Carl when he attacks a claw machine and smashes the glass with his fist and later falls for Frylock when he starts punching Carl's coffin at his fake funeral.
She finds Carl sexually attractive and has a high sexual labido.
In a summary of her own words, "He bugs the ass out of me."
Darline is, by many standards, an unfit parent and does not seem to hold any parental instincts. She disregards her son's needs for her own, such as leaving him with a (bitter) Frylock for a few weeks while she has constant sex with Carl. She views her child as a burden and seems to care only for herself.