Pokémon Platinum features 2D sprites for characters and Pokémon, with some 3D elements in certain areas, such as the game's overworld and some battle animations. The graphics are an improvement over Pokémon Diamond and Pearl, with more detailed environments and animations.
Pokémon Platinum was released in 2009 for the Nintendo DS, and it features improved graphics compared to its predecessors. However, if you're looking for Gen 5 graphics, you might be thinking of Pokémon Black and White, or Pokémon Black 2 and White 2, which were released for the Nintendo DS in 2010 and 2012, respectively.
That being said, here's some information on Pokémon Platinum's graphics: