I think it might be fun to annoy the crap out of someone, but as far as actually winning the game it would be difficult. There isn't much prize draw, and with a max possible 4 Devo Spray you won't keep the lockdown forever. You could donk them if you are super lucky and just discard everything in their hand they could bench, but that requires a perfect hand for the first few turns, as well as multiple Devo Sprays, Puzzle of Time, and other such things. It seems like fun, but much too gimmicky and clunky.
My biggest problem when I've been playing Decidueye is the lack of a solid damage dealer in the decks I had it in. I've paired it with Trev-EX+Beedrill-EX, Tauros-GX, & Latios-EX (gotta get that donk) in the 3 main decks I play our favorite owl in, and it's remarkably hard in some circumstances to deal damage to garnish KOs sometimes. That might just be me and still learning how to manage my assets properly, but it's difficult in my experience.
I've said it in another thread, but there are 2 things I've played around with pairing with Decid (both are untested as of now though). Umbreon GX is the first one, and actually seems like it could put in some work and be fun. Shadow Bullet dealing 30 to something on the bench combines well in theory with Feather arrows all over the place, they don't share a weakness, and one of Umbreon's biggest faults is that it doesn't do a lot of outright damage (which Feather Arrow fixes again). One Shadow Bullet+Feather Arrow is a KO on an active Shaymin, so that's cool. It also allows for some fun plays with Max Potion: Strafe for chip damage, and switch in an energy-less Decidueye (because Decid often doesn't have many attached in the first place) with a Float Stone attached to tank whatever hit comes next. Then next turn retreat out to Umbreon again, Max Potion to heal Decid completely with no loss, and either keep the loop going or Shadow Bullet for the KO.
The other thing I played with on a drive somewhere, and seems like a terrible but potentially hilarious meme deck, is to pair Decidueye with Delta Plus Dustox. As reminder, that Ancient Trait lets you snag another prize whenever it KOs something. This pairing focuses on Dustox's second Attack, Wind Shard, which does 50 damage to one of your opponent's benched Pokemon, and the fact that both of them use FoGP to evolve fast. Whittling down with Feather Arrows first, and then bopping with Dustox could let you snag 3 prizes on a benched EX! It operates a bit like Meowth from Fates Collide that was mentioned in one of the articles recently, but with a more expensive energy cost (GCC), being limited to benched things, and an Ancient Trait. I think it would be WAY to clunky to take seriously, but it could be a fun one.
That's my thinking for now. Thoughts?