I found a fix for mine. It was two problems:
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As another poster mentioned, you have to have your character pose exactly match your mocap pose. The offsets between source and target poses are really strong. If your mocap has a stick-straight neck and your target has a natural neck, the naturalistic mocap will be additive to the character, pushing their neck further forward than you want. Same thing with the fingers-- stick straight mocap needs stick straight character.
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I had my finger chains defined as just the three finger joints and the metacarpals were left out. I needed to also go into the IK Rig, search for “meta” in the tree and right-click Remove. This stops the retargeter from putting wacky rots on the metas and leaves them in the offset aquired from the pose, as described above.