Your isometric 3D model is blocky usually because it has a low polygon count or lacks smoothing techniques (like subdivision surfaces) to soften hard edges.
Low-poly models use fewer faces, creating angular, blocky shapes—common in stylized work, but unintended blockiness happens when smoothing is skipped. Isometric views highlight these edges, making blockiness more noticeable.
To get a smoother look, add a subdivision modifier in your 3D software (e.g., Blender’s Subdivision Surface) or use normal maps to fake detail. Keep adjustments small to preserve the isometric style.