Problem is that Clefairy can also instantly evolve with a Moon Stone, and can benefit TM spam far better so that "intent" just falls flat right real hard. After all, the more common one doesn't have to be the best option, but when you made it so it is bad in too many ways possible to the point you really lean to the rarer one, that's just not helping the case.Jigglypuff is slightly easier to find than Clefairy so the intent was probably from a gameplay perspective of "here is the worse but slightly more common pokemon you can instantly evolve for a """"major"""""" power boost and spam tms with until it fals off and use someone else"
I found too many early route Pokémon, even a few rare ones, to end up not worth investment for right after early-game. It should be viable to at least mid-game. Raticate surprisingly can still do decently does to Super Fang, but almost everything else either falls right after the first two Gyms or isn't even seriously viable to begin with.
The easiness of the games' main story does have a perk of making it possible to beat it with any Pokémon, but the weakest links had to be overleveled or go for cheese to make it work.