I'd argue that there's nothing fundamentally wrong with Goodra as a Pokemon. Sure, giving it more offensive firepower would help, but it's clear that Goodra has a very distinct design approach in terms of battle and its niche is pretty clear: it's a special wall/tank. It has usable offenses but most importantly, it has unbelievably good Special Defense, which means it's designed to capitalize on the Dragon-type's innate defensive qualities, moreso than other Dragons who use Dragon's raw power and neutral coverage plus boosting moves to be destructive forces (Salamence, Garchomp, and Hydreigon are all built on that approach). Goodra, on the other hand, focuses on the type's uniquely great defensive qualities in that Dragon resists four elemental types and only has three weaknesses, and its raw bulk plus access to Poison moves gives it a better chance against Fairies than most other Dragons would, plus Goodra also boasts wonderful coverage, a bit of utility in Dragon Tail and Toxic/Infestation, and some clutch abilities in Sap Sipper and Gooey, as well as Hydration for rain situations.
When you look at it, its niche is clearly to switch into and check/counter nearly every special attacker and serve as a catch all sponge against a large myriad of strong special nukes, especially against Sun and Rain teams with their Water and Fire-types as well as Venusaur. And it has all the tools at its disposal to do that, except one: reliable recovery. That is where Goodra falls short in Singles: it's a tank/defensive dragon, but its ability to act as a tank to counter special attackers is hampered by its inability to reliably heal itself with Recover (considering Gastrodon gets this, why Goodra doesn't is beyond me). Which means it cannot repeatedly switch into the stuff it's supposed to in the long term as not being able to heal means it has limited longevity, which is an important tool for a Pokemon with Goodra's design philosophy to have. This is why even stuff like Hydreigon and Salamence manage to overshadow it: the big thing isn't that they have more power, per se, it's that they can heal themselves. The fact that they have Roost means they can stay alive for a longer period of time, which is what gives them the edge in Smogon 6v6 Singles metagames. Goodra's inability to heal itself to that extent means its ability to act as a tanky Dragon is hampered.
Arguably, Goodra would also benefit from having a second typing like Water or Poison which would help it even more. Dragons always benefit when they are dual typed, and Goodra having a second type would help it immensely.
I also alluded to it in the OI Discord a few days ago, but I also firmly believe had Goodra existed in Gen 5, it would've been an excellent Pokemon. It would've been a staple on rain teams, and with Hydration+Rest in permanent rain it has immense longevity as a tank that can wall the likes of Thundurus and even Tornadus to combat Rain teams and also have awesome coverage and utility to utilize as a defensive Dragon on rain.
In fact, despite its mediocre Smogon record, Goodra actually has a surprisingly solid track record in VGC on the other hand, which is a more fast paced, 4v4 Doubles metagame as anyone who has knowledge of VGC would know. Goodra performed wonderfully in VGC 2020 as a powerful Special Tank with awesome coverage, especially in tandem with Comfey who could use priority Giga Drain to activate Sap Sipper and boost Goodra's power, and most of all, Floral Healing to give Goodra priority recovery enhancing its longevity greatly, while Goodra could use its awesome coverage to gradually sweep through stuff and its typing allowed it to check a lot of stuff like Charizard, Venusaur, and even Indeedee.
It also had good performances in VGC 2014 and 2017 where its beautiful special bulk and typing made it a great blanket check to so many Special Attackers like Charizard Y, Greninja, Gengar, Ludicolo, Zapdos, Porygon-Z, Thundurus, Nihilego, Tapu Koko, and sooo many other Pokemon all in just one Pokemon, which is an impressive feat in and of itself.
It's worth noting that Gen 6 is where the VGC metagames really started to take off, and I strongly believe that Goodra was likely designed with a 4v4 Doubles environment in mind (Kommo-o and to an extent Dragapult likely were too). It has a strong niche to perform in Doubles and it can do that niche really well in said environment, but not giving it any reliable way to heal itself on its own hampers it significantly in Singles. Had it had reliable recovery or if Rain were still permanent (so it could use Hydration+Rest), it would be a much better Pokemon for Smogon Singles environments.
