I think it's Gamefreak/Pokemon trying to reiterate that all pokemon are equal whether they are big or small. So by putting a cute, seemingly harmless pokemon as the strange #000 numbered legendary, it adds a little diversity to the game rather than seeing another strikingly powerful looking pokemon in the games. We have our cute looking legendaries already, such as Shaymin and Mew, but this guy looks a lot more ordinary.
I would also like to point out that it looks like you get him at the start of the game, look at the pokedex and notice, you have caught him as well as having your starter pokemon with very few other pokedex entries, thus proving that it is early in th game. Then again, it is an event, but possibly one you can activate from the start? Or of course, my theory could just be silly and they were just showing it that was as example. I think it's a pretty cool concept regardless, we have enough powerful looking legedaries, why not have a few more very powerful ordinary looking ones?
