Hi camp-education,
To "bake" a joint rig, Alembic (abc) is not the way to go. It has no concept of Joints. But yes, it turns Joints into "Nulls," and one could retrieve information from there, which feels like Destroying information while being aware of it and then putting a lot of effort into restoring it.
Why do you want to bake it? Your question is short and has very little to go on.
For external use, the FBX is a classic; for internal "baking," the Animation Clip?Motion Clip is the way to go. I place the Mixamo rig typically under Null, so I can rename it and have each time the same hierarchy, as that is important to work with Animation clips.
All the best