Mono-Grass Types are known to get the shaft as seen with Sunflora, Meganium or Maracus. Carnivine may not have as bad of a bulk as Sonflora and perhaps it has the best ability of the bad grass types making it immun to most entry hazards and Ground-Type. Though it's attack is just average while all its other stats are below average besides Spa which still isn't anything to be amazed about. Despite it being more of an offensively orientated Pokemon, you don't want to set up Trick Room in a serious battle to make this thing useful and its coverage isn't even good.
On the special side you only have Stab like Giga Drain or coverage in Sludge Bomb and Hidden Power while physically you have only Stab + Dark moves + Normal moves. There are plenty of better options for an offensive Grass Type. Even something like Victreebel's pre-evolution Weepinbell has less opportunity costs over this thing and Victreebel is known to drop bottom tiers most Generations.
The only recent relevant buff Carnivine got for Singles is Defog, but good luck getting it off.
Also check out this video about DP Remakes for more:
https://www.youtube.com/watch?v=WBgbji9raC8
