Mixamo is not a retopology tool, but tools like Blender (with its Retopoflow add-on) or TopoGun are ideal for creating retopologized face models compatible with Mixamo’s rigging/animation.
Retopologizing faces for Mixamo needs evenly spaced topology—especially around eyes and mouth—to support smooth animations. Blender’s free, built-in tools or TopoGun’s organic-focused features help build this clean structure.
For beginners, start with Blender’s Retopoflow add-on (free for Blender users) to practice on simple face models. Then upload a low-poly, UV-unwrapped face to Mixamo to test rigging compatibility.