Is the Canon EF-S 55-250mm f/4-5.6 IS STM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 250mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF-S 55-250mm f/4-5.6 IS STM Lens set to 250mm f/8.0 sharper than the Sigma 18-250mm f/3.5-6.3 DC Macro OS HSM Lens set to 250mm f/7.1? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.