Howdy, everyone! Many years ago, I made a post about missing/messed up textures when importing .obj files from Anim8or into Mixamo, and just last night, I FIGURED IT OUT!! It's a bit strange as to how it works, but bear with me.
-Your model exported as a .obj with the .mtl file that exports with it
Note- You gotta mess around with the specularity of your textures, they can make it look like you have no textures, or that there is a slight shine that is your texture. Try 0.06 for a nice leather shine, 0.01 for a nice rough plasticy shine, 0.1 for hard shine and 1 for blasted lmao
Okay, so, step one:
Zip the .obj and .mtl files into a folder. Name it your model's name.
Highlight the textures, right click and press copy.
Click into the zipped folder and paste your textures. (its weird, but that's how you've gotta hit this thing.)
Open Mixamo, go to upload character, and drag your zipped folder into the part where it says, 'Select character file'
If you have any issues, please comment and I will help you figure it out!