With the Snivy Community Day putting the Grass-type Pokémon back on the map, you may end up wanting to use or counter Serperior in the Pokémon Go Battle League in the future.
Serperior isn’t actually frequently used outside of the Great League, and even with the addition of Frenzy Plant, that is unlikely to change because of its poor overall stats.
At its peak, Serperior can have Max CP of 2,574 at Level 50 and 2,277 at Level 40 depending on where you plan on using it. Its stats are lackluster, with 161 attack, 204 defense, and 181 stamina.
Prior to the addition of Frenzy Plant, most players would likely be running some combination of Vine Whip and Grass Knot to maximize speed and damage output, or Iron Tail and Grass Knot for defensive uses in PvE scenarios. Leaf Tornado is a decent pivot, but you lose damage output compared to Grass Knot in that Charge Move slot.
Nothing much changes when you enter the PvP setting either, with the favored moveset still being Vine Whip and Grass Knot, though being able to have Aerial Ace in that second Charge Move spot gives Serperior options against other Grass-type and keeps it from being dead weight.
That staple combo gives players around 12.8 damage per second, which is respectable, but also the highest you can reach without relying on Frenzy Plant.
As a whole, Serperior is far from the worst Pokémon in the world, but being a pure Grass-type with defensive stats doesn’t do it any favors. It has poor overall stats, a shallow movepool, and is outclassed by other Grass-types without even taking into account dual typings.
For the Great League, you can get by with using Serperior on your team if you lack any better options since Water-types typically run rampant. However, once you enter the Steel and Dragon-type infested lands of the Ultra and Master Leagues, you should probably just look for better options.