On "Seinfeld," the "woman in the blue sweatpants" in "The Good Samaritan" is Becky Gelke. Jerry has noticed her from across the street and knows her only by that descriptive nickname before learning her identity.
Becky appears in the 1992 episode "The Good Samaritan," the 37th episode of the series. Helen Slater plays the character, and the role is uncredited. The episode's main plot follows Jerry after he witnesses a hit-and-run accident and becomes involved with the attractive driver, Angela.
A common mix-up is confusing Becky with Angela, because both women are central to Jerry's romantic attention in the episode. Angela is the driver Jerry sees after the accident, while Becky is the mysterious neighbor he has admired from a distance. The blue-sweatpants description is therefore a clue to Becky Gelke, not a general description of the episode's female characters.