No, Pong Krell was not a Sith Lord. He was a fallen Jedi who was pretending to still be a part of the Jedi Order and the Republic. He planned to sabotage the Republic forces and show his allegiance to Count Dooku in hopes that he would become his apprentice. At this time in the Star Wars universe, the Rule of Two was in place. This rule stated that there could only be two Sith Lords at any one time, a master and an apprentice. At the time, Count Dooku was a Sith Lord apprentice while Darth Sidious was his master. If Pong Krell were to become a Sith Lord, it would mean that Dooku would have to kill Sidious. This is how the Sith worked in order for them to remain as powerful as they could be. The Rule of Two was started by a famous Sith Lord known as Darth Bane.