After Effects quality drops
Make sure that resolutions match.  Look at the properties of the source clip, make sure that all of the main parameters (height, width, pixel ratio, frame rate) match for your composition and your export.  If it is having to interpolate pixels (as a result of non-divisible height or width or a difference in pixel aspect ratio) or interpolate frames (as a result of a mismatched framerate) then you are going to see a noticeable drop in sharpness.

Also check to make sure that render options are all set to 100% and that Best quality is selected on the render output.  It is possible to run renders in AE that don't process every pixel to speed rendering for drafts.  This could be what is happening as well.  If you can provide more detail about the composition, render settings and source material, then we might be able to help more.