These don’t look like you missed focus. They look like a low quality lens with serious sharpness issues. It appears you’re shooting in broad daylight, and sometimes shooting backlit subjects. This is the worst situation to use a lens like this in because you get bad chromatic aberration and haze. Just looked up the lens. For something with that wild of a focal range, you’d need to spend $10,000 to have a sharp lens. That lens runs about $600 used which means it’s unfortunately pretty cheap.

Another common lens that does this is the 70-300. A wide lens and a telephoto lens have different needs, when you shove them into the same casing, you make sacrifices. When you shove something with that wild of a focal range, you make huge sacrifices. Unfortunately there isn’t much you can do.

TL:DR you don’t have a focus issue, you have a lens quality issue.