Should I return/replace my lens? EF-S 55-250mm sharpness/focus

I use autofocus on the center point, and I do believe that I use settings that are generally supposed to come out with sharp photos, such as an appropriate aperture + 1/FL, etc.

It's the shutter speed, not aperture, that's supposed to be 1/FL. It would be rather difficult to set an aperture of f/100 on a 100mm lens, for example, but it's easy enough to set a shutter speed of 1/100th of a second.

Autofocus doesn't work well on smooth surfaces, because it works on edges. A striped shirt, for example, makes a good focus target. Also, try to fill the AF target box with the thing you want to focus on - it can easily get confused if you are focusing on something small. If your subject isn't wearing stripes, then place the focus box on an edge between your subject and a contrasting brightness background - like a light coloured subject on a dark background, or a dark coloured subject on a light coloured background - that will give your AF the best chance of nailing focus.

The 55-200mm zoom is not a high quality professional lens, so don't expect perfect results. You can get good images from it, but there's a reason people spend a lot more on lenses like the 70-200mm f/2.8L IS (as someone facetiously suggested ). Bear in mind, though, that the L series lenses are most needed when trying to get the shot under difficult conditions. If you stick to relatively easy shooting (lots of light, good contrast), you should be fine.