That plugin is licensed under the GPL (GNU General Public License), so you can modify it as you please. You can even redistribute your modified version, as long as you do so under the GPL and adhere to its requirements.
That said, even if it wasn’t, I believe you would be allowed to add CSS, since you are not even modifying the plugin in any way. I personally would look for a way to actually remove that link from the output, though, not just via CSS.