When you say "focus" I'm wondering if you are referring to the zoom range?
The focus should be fairly even across the entire field. Technically image quality for any lens is usually a tiny bit better toward the center and a tiny bit degraded toward edges (and high end lenses do a better job of controlling this). Performance will also vary even within a zoom lens based on where you are in the zoom range (focal length range) and focal ratio.
If an optical element were to somehow become askew (I suppose a lens would need to receive a pretty good hit to do that) then it can produce a non-flat field where one side of the field is in focus and it goes out of focus on the other side (and this could happen in any direction -- including diagonally.)
The image sensor within the camera has adjustable shims -- but factory adjustable. These normally never need any adjustment once set by the factory. A moderately well-known incident happened in the photography community when Nikon had a factory calibration target which was not properly set up and an entire range of serial numbers was shipped to consumers with the sensor out of adjustment -- causing imaging to be out of focus on one side of the field (Nikon had more than one line doing calibrations ... so not every camera was affected... just every camera that was adjusted through that particular focus calibration target.) To my knowledge, Canon has never had such an incident.
If you are looking for a lens with a broader zoom range, Canon makes an EF-S 18-135mm f/3.5-5.6 IS (and now they make a new IS STM version.) The lenses with the "STM" suffix have Canon's new stepper motor. The motor is faster and quieter than the original motors, but also most of these lenses have had their optics improved. The STM lenses tend to cost about $50 more than the non-STM version of the same lens, but are well worth the extra $50 in terms of optical performance and focus performance.
Canon also makes an EF-S 18-200mm f/3.5-5.6 IS lens. These are sometimes called "super-zoom" lenses because the zoom range is so broad (there's more than a 10x difference from 18mm to 200mm). All lenses are a game of trade-offs. The advantage of owning a camera that has removeable lenses is that you can remove one and attach another which is more optimal for the task you want.
Lenses that do not zoom (also called "prime" lenses) can be well-optimized for their focal length and usually offer a substantially lower focal ratio (which means they gather a LOT more light and can also provide a narrower depth of field).
Zoom lenses with less ambitious zoom ranges... where the smaller number divides into the larger number only 3 times... perhaps 4... such as a 24-70mm or a 70-200mm ... tend to be able to achieve better optical quality.
The super-zooms tend not do as well in optical quality (the Canon 18-200 does well in the lower half of the range but the optical quality degrades slightlyl as you get to the longer end. The optical quality from an 18-200mm will not be able to compete with the optical quality of a 70-200mm when making the comparison near the 200mm end.
If you're looking to replace your 18-55 with something that offers a bit more range, then the EF-S 18-135mm f/3.5-5.6 IS STM is probably one of the best options. I would probably select that above the 18-200mm.
If someone tells me they are going on a vacation and they only have room to bring a single lens... then the 18-200mm wins out due to the convenience of the range.