Legal depends on details. There are copyright, patent, and trademark laws that apply depending on how the cloning was done. Ask a lawyer for details.

The idea of a game is not protected. The specific graphics, dialog, script, and code are protected. Save the princess in an 8 level game is generic but just a few more details copied and suddenly you are an illegal clone of the NES Mario brothers script...

If you are worried talk to a lawyer about preemptively registering your copyrights (cheap) and trademarks (more expensive).