It's ironic. For years Agatha had treasured her locket, unaware that it had been crippling her Spark. Now that she has it back, she'd dearly love to take it off again, lose it forever.
But she doesn't because somehow, that locket is keeping her mother's spirit from possessing and controlling her body once more. And now, her Spark has grown too strong for the locket to cripple her mind.
Is that what Uncle Barry had really made the locket for? Not to suppress her Spark, but to keep her from being used by Lucretia? Agatha hopes to ask him, someday.