In the main game series, the attack Low-Kick is a Fighting-type move that deals additional damage the heavier the opponent is. Pokémon Go uses Generation VI rules for moves and attack types, so Low-Kick's description would be.
A powerful low kick that makes the target fall over. It inflicts greater damage on heavier targets
In Pokémon Go, the attack does not contain the description pertaining the weight, as listed on Pokemon Go Database
A powerful low kick that makes the target fall over.
Now that Tyranitar exists in Pokémon Go, this Pokémon stood no chance against Low-kick in the main game series. The reason due to being a dual-type of Rock/Dark, which makes this a x4 weakness to Fighting while being an extremely heavy Pokémon.
With Low-Kick being the ideal move against Tyranitar in the main game series, does the Pokémon's weight affect the damage dealt in Pokémon Go?