In Crystal, it is impossible for it to have perfect DVs and be shiny. However, if you’re using the VC version of Crystal and transfer a shiny Celebi to Bank, then it’ll retain its shininess while rerolling its IVs, which I guess gives you a one-time 1/1,073,741,824 chance of getting a 6IV. (Yes, even with 3 IVs guaranteed, the way Gens 6 and 7 generate IVs ensure that a 6IV is always just as rare, regardless of whether 0 or 5 IVs are guaranteed). So a 6IV shiny Celebi is still impossible outside of hacking (or exploiting Crystal’s cloning glitch, I guess, but even then, it feels unlikely).