Platformers are not a very popular Genre today (compared to other), Mario is a different Story because it is a Mario its an Icon.
LBP is selling all right, but look how much money was put into Advertising and How much hype itd had.
Look at sales of recent platformers of the Wii all decent: Wario Land, De Blob etc. etc. but they will never produce sales of shooters etc. etc.
HONESTLY, Why do you feel the need to bash on LittleBigPlanet? It can't just be considered a good game. It's an over advertised game. Just leave it alone for christs sake.
Otherwise, Platformers are NOT dead. This is just by a very minor company that doesn't exactly scream "amazing" with their projects in most cases. Other then that, I do agree with your last sentence.