The idea of status effects seems good but isn’t this particular effect mechanically similar to the defense drop from something like acid spray. The extra fast move damage per turn from lowered defense would amount to the same thing .
Only difference I can see is that acid spray debuff goes through even if shielded and you have specified the damage as poison type instead of whatever fast move is being used.
The main stat effect which would require a new mechanic is something to make opponents sleep or be paralyzed as we have nothing to skip turns as of now.
Actually even those are not needed because since there is no speed stat in pokemon go it seems that there are only two metrics to consider in pvp
Damage done before death or TDO
and damage done before opponent hits charge
All types of status effects will increase both of those metrics and would be the same as an attack buff or defense debuff with minor differences because they will be applied to different pokemon(Yours or your opponents) and since its percentage buffs it will depend on individual stats.
I may be completely wrong in the last para as I have very little experience with msg though. Feel free to correct me if so.