No Giratina is the representation of antimatter (officially confirmed by the creators) people are so fanatic with this idea that giratina is a demonic being that they forget that all of this is just speculation and does not have any official evidence to suport it, plus theres a lot more going on with it's design than just "fallen angel", and the hole thing of the 666 makes no sence due to the fact that theres a bunch of other mons with the same pattern so its not exclusive to giratina and with the revelation of its beta design we can see that giratina was allways meant to have 6 legs, so its more of a coincidence than an actual intended thing.
While I dont blame people for thinking this, people sometimes forget that Pokemon is a game meant for all types of ages including children, and the concept of the devil is not that kid friendly, plus satan is an extremely controversial figure and I doubt that GF wants controversy, while I dont disagree that giratina may have some inspirations with the concept of a fallen angels banishement story, there is clearly a lot more with its design than just meets the eye, and this over exaggerated connections with satan that Pokemon fans do ruined the character and what it was meant to be.
i now understand why parents were afraid that Pokemon was a demonic game and I dont blame them