No, Unown and Celebi are not linked in any way.
Ways to get Celebi:
Ruby Obtain from Nintendo Event
Sapphire Obtain from Nintendo Event
Emerald Obtain from Nintendo Event
FireRed Obtain from Nintendo Event
LeafGreen Obtain from Nintendo Event
Colosseum Trade from Games that have Received Celebi from Event
XD Trade from Games that have Received Celebi from Event
Diamond Transfer from GBA via Pal Park
Pearl Transfer from GBA via Pal Park
Platinum Pal Park like Diamond and Pearl
HeartGold Trade from DPt
SoulSilver Trade from DPt
- Copied from Serebii.net
So the only way to get a Celebi in Diamond is to trade from somone who has one, or migerate one over from Hoenn or earlier Kanto. (Not in generations due to Pokemontrainer probably not understanding them).
