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 100mm f/16.0 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 100mm f/16.0 sharper than the Tamron 100-400mm f/4.5-6.3 Di VC USD Lens set to 300mm f/6.3? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.