So I checked out the Mixamo site. One thing that creates an issue is that the downloads are in FBX or DAE format, not BVH which is what MB-Lab reads. This is why they do not work in Blender, the MB-Lab code relies on the BVH format for re-targetting the armature. In order to use the Mixamo animations out of the box would require a huge code re-structure, something I am not able to do at this time.
There is plans to change the armature slightly, there were some conversations in this thread regarding that. This would need to have some code rework too since it works hand in hand with the JSON files associated with the project.
We are planning on more base models, the problem is that we need to reverse engineer the JSON files and the code first, then build tools to write the same information for the new base model. We are not there yet but we are researching this.