I think a T an P was created because those characters are expendable as comic relief. and you're right . the lions aren't that funny.

not to mention if they had lions cubs whether it be Simba or Nala or possibly Kopa there'd be a lot of people doing the voices ..

also another thought. maybe they just used T and P so they wouldn't get in any more trouble with the Kimba people . seeing as there were no Meerkats (save some background Meerkats in the Jungle Emperor movie) and a recurring Warthog character. ( although there was a warthog character in one of the Kimba episodes)