Evaluation and application of sharpness and noise reduction can only be done accurately at 1:1 (100%) view, where one image pixel is represented by one screen pixel. Any other magnification will be inaccurate and misleading, because of image scaling.
Once I'm satisfied that an image is sharp (at 1:1), I don't care if it looks slightly unsharp at other magnifications, or in Library, because I already know it is sharp.
If some photos are really unsharp in Library, you can try rebuilding the previews.
Select the images, and go to Library > Previews, and choose Build standard sized or 1:1 previews.