Does the Pokémon's weight affect the damage for "Low-Kick"?
Solution 1:
From the most recent APK datamine, Low Kick has a set damage number given and has a defined DPS. Due to this, it seems like Low Kick will not take Pokémon weight into account when determining damage:
207: LOW_KICK(FIGHTING). Fast. Dmg: 6. CD: 1100ms. DPS: 5.45. NRG: 8. EPS: 7.27. WndSt: 300
I believe the fact that the weight part of the move description was left out also alludes to this being the case. This is not the first time a move with secondary effects in the main series has been missing its effect (moves that always cause a critical hit, moves that raise/lower stats, etc).