Not very viable, because while Contrary + Leaf Storm is excellent in singles, in doubles it isn't exactly very effective. It is frail and weak to Heat Wave and Blizzard, and is outclassed by Amoonguss, Kartana, Whimsicott and Ferrothorn. Ferrothorn is incredible in rain, as it becomes nearly unkillable. Kartana exerts a bigger offensive presence because unlike Serperior its attack stat is already amazing without a boost. Amoonguss has the excellent Spore and Rage Powder and has a great defensive typing, as well as nearly infinite longevity between Giga Drain, Synthesis, berries, and Regenerator, as well as Clear Smog to remove dangerous stat boosts. Serperior's access to Glare for speed control is almost completely invalidated due to Amoonguss very occasionally running Stun Spore, Kartana's access to Tailwind, Whimsicott's Tailwind and Trick Room, and Ferrothorn's access to Thunder Wave. It is destroyed and walled by VGC staples like Incineroar, Mega Charizard Y, Genesect, Kyurem-B, Volcanion, Amoonguss, Mega Metagross, Mega Salamence, and others. It also struggles mightily in Trick Room, where it is demolished by Mega Camerupt and Chandelure's Fire Blasts. It also has very a tiny coverage movepool, and its role as a Dual Screener is taken by the more useful Klefki.
However, it does have a fast Taunt going for it (although like SleepyDreepy said Whimsicott's Taunt is faster). It also has access to Grass Pledge to combo with other starters, and Leech Seed is good too. Being able to freely switch into Snarl and Icy Wind(Icy Wind is debatable) is also amazing.
Hope this helps!
