Consider moving Nidoking from C+ to B-? I think Nidoking brings more to the table than most of what's currently in C+ and is more comparable to the Pokemon sitting in B-, if not better than some of the things already there.
Nido's main niche is being an offensive check to Clef. Considering how defining Clef is, this is a notable niche. Nido's immunity to Thunder Wave helps with this greatly, as Clef can usually threaten to paralyze most wall breakers. This helps distinguish Nido from other more powerful and effective breakers, even if only slightly. Aside from this, Nido does a nice job threatening fatter builds. Great coverage between Earth Power, Sludge Wave, Ice Beam, and Flamethrower allows Nido to do a number to a lot of the slower Pokemon found on these teams, such as Skarm, Ferro, Amoonguss, Tangrowth, and Tran. These are all relevant Pokemon in today's metagame, and Nido does a nice job abusing their presence to pressure the opposing team.
While Nido is mainly known for the above two traits, there are several other things it's able to do. One is soft checking offensive electrics like Mega Manec and Thund-I. The main advantage Nido has here over other grounds like Lando-T and Chomp is the lack of a 4x weakness to ice. Why is this significant? While this does require prediction, consider switching into a predicted Volt Switch or Thunderbolt from those electrics. Nido can tank a HP Ice from both and threaten to kill them with the appropriate coverage move. Unless they are using Scarf, Lando-T and Chomp can't do the same because they will drop to HP Ice afterwards. This is one area where Nido separates itself from the usual grounds of OU. Being able to check non-EP Mega Diancie and Loom is also nice, and this all combined prevents Nido from being a complete liability against the offensive teams it struggles more with.
Nido does have its problems. It has a hard time getting on the field due to its frailty and weakness to common types like ice, water, and ground. Its lackluster speed also leaves it vulnerable to revenge kills from several dangerous Pokemon, including Exca, Weavile, Keld, Mega Zam, and many others. Despite this, one thing to consider is how Rotom-W and Torn-T are two of Nido's best partners. They are both great Pokemon, provide chances for it to come in for free, and cover a good amount of threats for Nido. Rotom-W in particular has been seeing more use lately, and its tendency to draw in things like Ferro, Tang, and Clef is taken advantage of well by Nido. VoltTurn in general is really effective right now, and Nido pairs nicely with two of its common users.
Overall, I think Nido has a few things going for it to warrant a rise. It's not some dominating force, but when compared to other C+ Pokemon like Staraptor and Tyrantrum, Nido provides more valuable niches to the average team. After looking at some stats from tournaments (
here and
here) and speaking to a few people, Nido has seen more relevancy in tournaments compared to other niche Pokemon in the C+ and B- rankings. Even if the sample size isn't as large as the ladder, this does show (even if minimally) it's a more appealing option to some players compared to other Pokemon sitting near Nido's ranking.