Fred Wolf Films owns the actual episodes, and it is still an active studio:
http://www.fredwolffilms.com/
I think Lionsgate currently holds the "home video" rights. Since Nick/Viacom owns the actual franchise, I expect there is still some obligation to Lionsgate to credit or acknowledge them.
Nothing prevents Nick/Viacom from using the likeness or characters from the old show (Mirage Studio artists did they all the time and were often commissioned to do Fred Wolf style artwork), but they are fairly careful to avoid using any actual excerpts from the episodes.