Lenses with big zoom range like the 18-250 are known to performer poorly at optical quality, especially at the longer ends. Even a kit will often be sharper. There are good and bad constant aperture lenses too.
To me, if I shoot portrait I'll use prime lens. They gave me exceptional image quality (not to mention the great backgrd bokeh). For normal shoot like street, I'll use the fast zoom lens. And for travel, long zoom range. So, to me what really matters is what lens you prefer depends on what do you want to shoot.
Most fix focal length lenses (prime lens) are sharper when compared to zoom lens but that was not always the case though. Anyway, if sharpness is the only thing TS is looking at, then... yeah, in general prime lens is sharper.
A zoom lens (especially those with much bigger zoom range) will have to have some compromise in IQ... which in my opinion can actually be corrected to a certain extend during editing so it is okay.
However if TS is actually asking about fix and variable aperture, then it is a different matter altogether. In actuality, I do not believe that fix aperture or variable aperture would have much to do with sharpness of the photo, although in general... a fix aperture zoom lens is always more expensive and 'premium' then a variable aperture zoom lens of similar zoom... and for that type of money, people would definitely want something that is sharper, more contrasty had better looking bokeh as compared to the variable aperture lenses.
Apart from the discussion of optics, just make sure the lack of sharpness is not due to handshake at the 250mm end of things.
Poor stabilisation during shooting gets magnified at the tele end of your zoom.