Your blend shapes 3D model is stiff in Auto-rig usually because the blend shapes don’t match the base mesh’s vertex topology or the rig has uneven weight painting.
Blend shapes need identical vertex count and order to the base model—mismatches prevent smooth blending, causing rigid movements. Uneven weight painting means bones don’t influence vertices naturally, making deformations look stiff.
Fix this by checking your blend shapes share the base mesh’s vertex structure, then use soft selection tools to smooth weight painting for more fluid motion.