When performing a test to determine X its best to remove all the variables you're not testing for.
If you're wondering about the sharpness of your lens:
- Shoot on a tripod
- Use manual focus with live view for critical focus
- Use mirror lockup
- Shoot in RAW
- Use a low ISO, at a reasonable shutter speed
- Turn IS off
The problem with your example is it looks like your focusing was off (the 'B' looks much more in focus) which could mean you focused and then moved the camera slightly, the AF mis-focused because of a myriad of reasons, in camera processing did something weird, your body has back focusing problems...
Some of those things are your fault, some of them are the camera's fault and some of them are both your faults but if you want to determine where the cause of the sharpness problem is you need to narrow down the variables and test each thing individually.