It is possible to create categories in the Material Browser of Inventor. However, it's not possible to create subcategories to get a better and deeper structure. It would be very helpful to separate different types of steel or plastic into their subcategories.
This is already done in the Autodesk Material Library for Metal/Iron.
In the Asset Browser you can see an even further separation into multiple subcategories for different kind of materials.
This idea has been submitted and archived here: Ability to create subcategory in the Material Browser - Autodesk Community
It has been discussed here: Createing af subcategory in the Material Browser inventor - Autodesk Community
here: subcategory - Autodesk Community
and here: Material-Browser: Unterkategorie? (*.adsklib) (Autodesk/Inventor) - Foren auf CAD.de