Is the Sony FE 135mm F1.8 GM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 135mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Sony FE 135mm F1.8 GM Lens set to 135mm f/2.8 sharper than the Sony FE 135mm F1.8 GM Lens set to 135mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.