The Distortion World (Japanese: やぶれたせかい Torn World) is the name given to an alternate dimension in Pokémon Platinum and the true home of the legendary Pokémon Giratina.
As far as the games, anime, and manga go, no there is not any other Pokemon other than Giratina in the Distortion World.
There is most likely Pokemon other than Giratina in the Distortion World.
The Distortion World is some sort of alternate dimension.
According to Dictionary.com, alternate (as a verb) means to change back and forth between conditions, states, actions, etc. (From my understanding, alternate means opposite.)
For example, lets say Sinnoh is the "good world" and the Distortion World is the "bad world." Lets also pretend that every Pokemon is a "good guy."
Since everything in the Distortion World is bad. Because, the Distortion World is an oppisite dimension our bad things their good things and vice versa. So, the Distortion World is basically an evil world, the good Pokemon from the "good world" are bad in the "bad world" because the "bad world" is basically the opposite of the good world.
But, there is no actual proof weather or not there are Pokemon in the Distortion World or not, only GameFreak knows.
Hope I helped!
(NOTE: I know Wikipedia is sometimes unreliable, but that's the only info I could find on what alternate dimensions are.)