By completing your regional and National Pokedexes to various degrees, you can speak with the Game Director of Game Freak (who appears in the Lilycove Cove Lily after you beat the game) to receive these diplomas, which will also be available as decorations for your Secret Base.

Diploma	How to unlock
Hoenn Pokedex Diploma	Capture all the Pokemon in the Hoenn Pokedex and speak with the Director.
National Pokedex Diploma	Capture all the Pokemon in the National Pokedex and speak with the Director.