When blend shapes look weird in manual rigging, the main issues are inconsistent topology between the base mesh and blend shape targets or incorrect vertex weighting.
Inconsistent topology—like mismatched vertex counts or edge flows—breaks smooth interpolation between shapes, while wrong weights make vertices move unevenly, causing stretching, bulging, or unnatural distortions.
To fix it, ensure all blend shape targets match the base mesh’s topology exactly, then use weight painting tools to refine vertex responses. This simple check often resolves weird results.