I am sorry, but that explanation does not make sense to me.
"Copy to System Clipboard" is essentially a screen shot of the currently visible part of the diagram. As such the created image cannot be "made clearer" no matter what you know about yEd's zoom factor. If you need "clear" images, either use "File" -> "Export" (which does allow control over image size/zoom factor) or use "Copy to System Clipboard" at zoom factor 1 (which you will get using the "Reset Zoom" action from the "View" menu or the corresponding tool bar button).