Sometimes when applying Mixamo animations to the character there is a slight stretching of the legs or the character’s feet seem to slide about and not look the same as when previewed in Mixamo. I have found the following workflow gets the animations 100% the same as how they look in Mixamo.
First export your character from Unity as an FBX (you can grab the FBX Exporter package from the package manager, just look under the Unity Registry section). Often these Synty characters have multiple skinned meshes, just drag a character prefab into your scene, unpack it completely, pull out a skin mesh and the root (skeleton rig), put the root under the skinned mesh, then right-click it and choose export as FBX. This should give you a nice simple rig to now upload to Mixamo
In Mixmo, press the ‘Upload a Character’ button and choose the FBX we just created. Mixamo should automatically detect the skin data and rig and skip the usual bone placement step taking you directly to a preview of your character in the viewport. Don’t worry if the materials are missing.
Now that you have your character set in Mixamo, you can preview the animations and see exactly how they should look on your character. This character will remain in Mixamo even if you close the browser.
Still, in Mixamo, search for a T-Pose animation and then Download and choose the ‘without skin’ option.
Take this downloaded animation into Unity, Set its rig options to Humanoid, and ‘Create from this model’ for the Avatar Definition. You can then press the configure Avatar button and choose to Enforce T-pose, but you should see that the skeleton is already nicely orientated.
Any other animations you now download from Mixamo should be brought in and set to Humanoid, but choose ‘Copy from other Avatar’ for their Avatar Definition, and choose the T-Pose Avatar we just created.
This should give you perfect results, as we are using a rig generated from Mixamo, made from the character we uploaded which is the same as the character we’re using in our game.