Is the Canon EF-S 18-135mm f/3.5-5.6 IS STM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 18mm f/3.5 test image. Observe the contrast and resolution of these results.
Is the Canon EF-S 18-135mm f/3.5-5.6 IS STM Lens set to 18mm f/3.5 sharper than the Sigma 17-70mm f/2.8-4 DC Macro OS Contemporary Lens set to 28mm f/3.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.