Ok, so I don't think I am the first one who has this problem but I can be the last one who has this problem. If your model downloaded from Mixamo acts strange, basically if it's glitchy and is kind of transparent and if you don't know what to do and how to fix it, you've come to the right place! So basically Mixamo is old and many 3rd party softwares and engines have updated their fbx file type and bones... the result is they no longer read the old types correct. Mixamo works with Unity for rigging and animation but you need to configure the materials manually to make it look correct in the final package. Now, let's get into how you can fix this problem (easily)

• Download blender, it's free!
• Open it, once downloaded.
• On the top left corner, click on 'File'.
• Click on the 'Import' option and import your .fbx or .dae file.
• Once imported, again click on 'File', but this time click on 'Export' option and export to your choice of file format.
• Once exported, open it in Unity.
• It should be now glitch free and should look just like it looked in Mixamo.

If it helped, please let me know.