Zygarde is basically outclassed by other dragons like Dragonite and Garchomp :/. It does get boosting moves that other dragons yearn for, but Skarmory and Togekiss just wall it, it has no fire STAB the other dragons usually carry. Has anyone tried Assault Vest Zygarde? It seems like it can run it effectively.
I don't think it's outclassed by Garchomp, as their only similarity is their typing. Garchomp is either a strong revenge killer, a SD sweeper, or a Wall Breaking mega. Zygarde, on the other hand, is normally a bulky
Coil or Dragon Dance user. Coil is not learnt by any other pokemon, so it certainly isn't outclassed there. Its Ddance sets are similar to Dragonite, but Zygarde's benefits are that it resists Stealth Rock and has STAB EQ instead of coverage EQ. Garchomp can also run a ParaShuffling set, which isn't outclassed at all by Dragonite due to Zygarde's better bulk.
Also, Togekiss
cannot wall it if it has Stone Edge, which most offensive variants do. Your point about Skarmory, however, is extremely valid, and is something that makes it, in at least one way, worse than other dragons.
Also, I do think Assault Vest
could be viable. You'd achieve insane mixed bulk, and be able to run every offensive move most Zygardes struggle to pick from. The only problem is that 100 Attack is mediocre when unboosted. I still think the Coil set is its best.