Tangrowth has insane physical bulk, it takes physical dragon attacks well enough. I'd argue not having a steel typing is what sets it apart from Ferrothorn because it doesn't need rain to beat things like ddnite, it can actually wall fighting types, etc. it has a resistance to earthquake which is actually a huge help. It might not have as many resistances, but it has the bulk to make up for it, not having to worry about these Common weaknesses a steel typing would give it is what makes it so great.
Is ice not a "Common" weakness?
I could see Tangrowth in C rank because it really does need a good amount of support to be effective. It needs a Special Wall of some sort to be paired with since its special defense is absolutely abysmal for a Defensive Pokemon. Plus it also needs some support in the form of a Rapid Spinner to make its Regenerator ability any useful.
I'd argue that any physical wall operates at it's best when paired with a special wall, and vice versa; there are few teams that don't opt to run a special wall, so I'd argue that this isn't necessarily a valid point to denote Tangrowth.
Additionally, Regenerator doesn't always need rapid spin support to be effective; it helps alleviate the pressure of hazards, especially if the opposing team is lacking spikes. While it appreciates spin support- as any wall in the game does- it's actually not as cramped by hazards as many Pokemon. It alleviates almost all of the damage it incurs by switching in to 3 layers spikes and stealth rock with regenerator, whereas Ferrothorn, whom we seem to inevitably use as the comparison for physically bulky grass types, would take over 25% of it's health. The only real difference in taking hazards well is Toxic Spikes, which are rare enough that I don't think it would warrant dropping Tangrowth a whole tier.
Anyway, what really differentiates Tangrowth, aside from sleep powder and lack of a four times weakness, as mentioned above, is the fact that it's set up fodder for less things; with HP Ice and Giga Drain, it can stop Sub Gyarados or Dragonite from setting up, something Which Ferrothorn can't always do (dragonite always gets sub up, Gyarados needs burn, but since it's partnered with Politoed so often, that's not a big deal). Ferrothorn relies overly much on leech seed and status to prevent set up, which often means it can't beat substitute sweepers.
Additionally, it should be noted that Tangrowth has other support options like knock off and aromatherapy to differentiate it. Knock off is especially interesting since it's one of the few viable users of knock off in the tier (the other being Tentacruel, who generally has better things to be doing with its moveslots).
However, Ferrothorn frankly counters so much more, so it's generally preferable, along with the obvious hazard support.
But all in all, I agree with B-