Everyone totally knows a Mrs. Howe. She's a totally cheerful old lady who loves being a mom and mothering everybody, whether they need it or not. That is, until she switches into Mother Bear mode. When Mrs. Howe intercepts Clarissa's first letter after the rape, she's a little less than sympathetic: "But pray, miss, don't make my Nancy guilty of your fault; which is that of disobedience" (296.3). (Brain Snack: "Nancy" is a traditional nickname for "Ann.")
In other words, Mrs. Howe feels bad for Clarissa, but she doesn't want her baby girl messed up with that bad influence—and she's willing to make a stink to prevent it from happening.
And this isn't the first time Mrs. Howe bares her teeth. As soon as Clarissa flies the coop from the Harlowe household, Mrs. Howe forbids any correspondence between Anna and her BFF. Mrs. Howe automatically sides with Clarissa's parents before getting the whole story. Narrow-minded? Maybe—or maybe she sees the danger in the situation and starts backing away before it's too late.
What this all comes down to is free will. Should girls get to determine their own destiny by choosing a husband, or should they not? Mrs. Howe's thinks not. She doesn't want to give Anna any hubby options beyond Hickman, because what if she goes for a Lovelace type? "I think it proper to bear that testimony against your rashness, which it behoves every careful parent to bear" (296.8), she tells Clarissa. Mrs. Howe sees Clarissa—and her model of self-determination—as the Contagion, and vigilance as the only cure.