For example, Goomy was described as the weakest Dragon-type Pokémon when Noibat exists.
Maybe by weakest they meant softest?
This is really more a case of "gameplay and story segregation" than anything else.
Statistically Noibat has the lowest BST of all Dragon-types, while Goomy has the standard 300 BST that base forms of pseudo-legendaries tend to have (we wouldn't get any exceptions until Dreepy two gens later), and thus Goomy has the higher BST by the sort of rule that its specific archetype follows.
Lore-wise, and in terms of imagination, Goomy could easily be perceived to be the weakest of all Dragons if you were to think about it in terms of hypothetical battle capabilities and intelligence. The thing is very slow, has no limbs, and has a soft membrane that is very easy to permeate. It can't hypothetically do much in the way of attack, it has no physical power as it's very small and light and as I said, it has no limbs while it's very soft, and it lacks much in terms of firing attacks in general. It has other problems that make it weak in nature: it's extremely reliant on its membrane remaining moist in order to survive, so it can only thrive in moist, damp climates: if that membrane dries out, it will become lethargic, become unable to breathe, and potentially die, adding to how weak it is as it's very prone to death by dehydration. Its horns, while very sensitive, also make it very poor in combat, as if anyone were to grab those horns, they would give Goomy an intense state of physical shock and cause it to become paralyzed for a while. In that sense, Goomy has very little going for it in terms of what it could do in action.
Despite Noibat's pathetic stats making it on par with a typical early game mon, hypothetically you could see this thing being able to do something to its opponents in battle, even if it's not strong. It can unleash supersonic waves that cause even a robust wrestler to become dizzy and suffer a headache, and because it's a bat, it can fly and escape its opponents if need be, making it able to survive well in the wild. It also has fangs and can bite as protection. So in that sense it could do some things hypothetically.
So in the end it's "gameplay and story segregation". With that said, what matters as much as stat total in terms of stats is how the stats are distributed, and Goomy notably really doesn't have *that* much more offensive firepower: much of its BST comes from its defensive stats, which would tie well into its in-universe behavior as while it can't fight very well, its body being made of water and soft allows it to become resilient and endure a few attacks every now and then. So it still sorta lines up well in that sense.