The Nidos in GSC are fairly similar to their counterparts in RBY, except they arrive slightly later in the game, evolve slightly later with a backtrack requirement, get their only good STAB move later, and have a slightly weaker selection of moves to pick from (but still pretty strong). They are still solid though, once they fully evolve. They still have Surf and Body Slam/Thrash (depending on gender and assuming you evolve them at level 23 or lower) and the elemental punches are probably going to get close to as many KOs as moves like Thunderbolt and Ice Beam did. They generally don't have trouble on most routes and their coverage allows them to contribute in pretty much every important Johto battle to varying degrees (aside from Morty, they're complete dead-weight there). It also helps that they gain experience relatively quickly especially early on when they're not strong and they don't have access to all of their moves yet.
I'm a bit borderline about A vs. B. I said B before though just because I prefer to go with the lower tier in cases where I'm not 100% certain. I think the lack of STAB and the fiddling around for the Moon Stone are significant enough flaws to keep them out of A, but they may be acceptable depending on how you look at it - their wide coverage allows them to get lots of KOs without STAB, and the Moon Stone backtrack, while inconvenient, is probably not going to take THAT long when you think about it.
It may also depend on how big you want A-tier to be. I personally prefer a smaller A but if we want to be less conservative and allow more stuff into the upper tiers then A may be OK for these guys. But for now I'll say B.
I'm a bit borderline about A vs. B. I said B before though just because I prefer to go with the lower tier in cases where I'm not 100% certain. I think the lack of STAB and the fiddling around for the Moon Stone are significant enough flaws to keep them out of A, but they may be acceptable depending on how you look at it - their wide coverage allows them to get lots of KOs without STAB, and the Moon Stone backtrack, while inconvenient, is probably not going to take THAT long when you think about it.
It may also depend on how big you want A-tier to be. I personally prefer a smaller A but if we want to be less conservative and allow more stuff into the upper tiers then A may be OK for these guys. But for now I'll say B.