Inkscape creates pixelated images whenever I copy and paste

This has to do with the clipboard and pasteboard syncing of OSX and X11. As pointed out in the Inkscape FAQ you can prevent this by unchecking X11 preferences > Pasteboard: "Update Pasteboard when CLIPBOARD changes". Downside is that you cannot copy anything from Inkscape to other OSX programs anymore. From OSX to X11 should still be possible.

If you use duplicate instead of copy and paste the problem doesn't appear in the first place, so you can keep the pasteboard syncing.