In this video, I cover the second version of the Credits level. Though you can collect Lums (possibly even reaching the maximum amount of Lums you can hold at one time), there doesn't seem to be any reward for doing so.
After you finish "The Land of the Livid Dead", you gain access to a more interactive version of the Credits level you see after "Shoot for the Stars". In the first version, you can only move around and watch. In this second and better version, you are free to destroy as many letters as you wish for your own amusement. (Note, however, that you can still die by being "crushed" at the top of the screen.)
I actually quite like these interactive credits, but their length unfortunately turns the novelty of this level into something rather tedious. If you don't want to watch the entire video, I don't blame you, but enjoy the music if you decide to stick around. (Also, The Magician doesn't say anything special after finishing "The Land of the Livid Dead" and the Credits. How unfortunate.)