As the somewhat crazy mother of Johnny Nunsuch, Susan causes a whole lot of problems for Clym and Eustacia. She really has it in for Eustacia, and she practices auditioning for The Crucible by constantly accusing Eustacia of being a witch (5.7.34).
Susan's hobbies include sticking Eustacia with needles (sure-fire witch detection), practicing voodoo, and worrying about her kid.
Susan represents tradition in a negative light in the novel. She also plays an interesting role in the relationship between Clym and Eustacia. By stabbing Eustacia with a needle in church, she actually helps to drive Clym and Eustacia together – Clym is so overcome with sympathy for Eustacia that he strikes up an acquaintance with her. And by allowing Clym to speak with Johnny after Mrs. Yeobright's death, she helps Clym discover Eustacia's actions in the whole affair. This discovery leads to the end of Clym and Eustacia's marriage. Susan, unsurprisingly, doesn't seem too broken up about that fact.
Clym started up, and Susan smiled in an expectant way which did not embellish her face; it seemed to mean, "Something sinister is coming!" (5.2.90)
Overall, Susan is not shown to be a particularly nice woman. She's driven by prejudice and superstition.