Your 3D model looks unnatural in auto-rig because inconsistent edge loops, triangles/ngons, or poor topology flow around joints prevent tools from distributing deformations naturally, leading to odd bends or stretching.
Edge loops must follow muscle/skin movement (e.g., elbows, knees)—auto-rig relies on this flow to predict natural bending; misaligned loops break this logic. Triangles or ngons disrupt flow, causing uneven stretching or pinching that looks fake.
Simplify topology to be quad-dominant and align edge loops with key joints—this helps auto-rig tools generate natural rigs without extra manual fixes.