Clarissa's aunt isn't a bad type. She sure sympathizes with Clarissa's situation, and she tries to help her niece whenever possible. Unfortunately, Mrs. Hervey isn't really willing to rock the boat to extract Clarissa from her upcoming marriage with Solmes: "Everybody is now convinced, that nothing is to be done with you by way of gentleness or persuasion" (59.14).
See, Mrs. Hervey would rather end the dialogue than deal with tons of trouble. "You must not answer me. There will be no end of that," she matter-of-factly tells her niece (59.15). The lady sure knows how to shut it down.