Successfully imported a MaterialX file and created a material variant.
Applying the material to a simple grid and using Tommy as a size reference.
Does my Scene Graph look correct if I simply want to be able to show hide Tommy in my destination DCC?

Did you intend to create the materials under a prim called "_"?

You scene will be problematic if you want to bring this asset in by Reference, because a Reference has to have a single prim that it points to, and it brings in that prim and its descendants. In your setup, "_" and "geo" don't share a common parent prim, so there is no prim in this asset where you can point the reference to bring in the geometry and materials. This is fine if you will be sublayering this asset into your scene, but that's a less common workflow for dealing with individual assets.

You might want to consider reading https://wiki.aswf.io/display/WGUSD/Guidelines+for+Structuring+USD+Assets, [wiki.aswf.io] which gives a lot of great advice about structuring USD assets.